I believe this isn't a client/server interop issue. I looked at this at Cray with 2.11 client & server. I don't have the detailed notes, but I concluded the issue was the kernel/VFS was presenting a new attr bit (ie Linux has added a new attr bit), and the MDS was complaining about it because it didn't recognize it. We decided it was harmless and low priority to fix. We initially saw this only on SLES, but I've seen it on CentOS recently.
Patrick, I think this is caused by Lazy Size on MDT feature which is just introduced in 2.12. So, not a surprise that 2.11 client doesn't have the problem.
No, 2.11 did have the problem, lixi_wc. I specifically looked at it and identified the bit as a new attr bit added in newer kernels. I would give more detail but I don't have my notes, since they're internal at Cray.
Ah, are you still able to get the exact error message?
For the flags of 0x60000, I think it is caused by LSoM because these two flags are LSoM flags:
#define MDS_ATTR_LSIZE 0x20000ULL /* = 131072 */
#define MDS_ATTR_LBLOCKS 0x40000ULL /* = 262144 */
I confirmed with the customer that they are using clients of 2.12 with Ubuntu.
I guess the 2.11 problem has a different flag value printed in mdt_attr_valid_xlate()? And then it should be caused by some kernel update of the bits.
I am not, and I just did the same looking as you... I agree, it must've been a different bit, or maybe I'm crazy and we were testing 2.12. I don't think so, but... Certainly, those are the LSOM bits.
And there are no new attr bits even in the current Linus kernel...
Hm. I will see if I can dig up the error from someone at Cray. Odd. For now, ignore me. 
It would be good to implement one or more of the mechanisms proposed in LU-11928. Probably the best would be a new OBD_CONNECT_LSOM flag send from the client and checked by the server, so that the client doesn't send these flags to old servers that don't support them.
Yingjin Qian (qian@ddn.com) uploaded a new patch: https://review.whamcloud.com/34343
Subject: LU-12021 lsom: Add an OBD_CONNECT2_LSOM connect flag
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: 11e2b730944d3afbe62f6559a316929e58927a20
Aurelien Degremont (degremoa@amazon.com) uploaded a new patch: https://review.whamcloud.com/34412
Subject: LU-12021 mdt: make useless error message a debug one
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: acf12670e6f9acd1fac9a999a3f37ccc5c5d3c6b
Minh Diep (mdiep@whamcloud.com) uploaded a new patch: https://review.whamcloud.com/34663
Subject: LU-12021 lsom: Add an OBD_CONNECT2_LSOM connect flag
Project: fs/lustre-release
Branch: b2_12
Current Patch Set: 1
Commit: a0e2915c67fed503cdadca3af23430095b7000c0
Oleg Drokin (green@whamcloud.com) merged in patch https://review.whamcloud.com/34343/
Subject: LU-12021 lsom: Add an OBD_CONNECT2_LSOM connect flag
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: fdd2c5d3a6e5f866e5ca887064e9ed37316af234
Landed for 2.13
Oleg Drokin (green@whamcloud.com) merged in patch https://review.whamcloud.com/34663/
Subject: LU-12021 lsom: Add an OBD_CONNECT2_LSOM connect flag
Project: fs/lustre-release
Branch: b2_12
Current Patch Set:
Commit: 03caa311ca920eb94cdb73935c46364e0ebe13a9