conf-sanity test_125 fails with 'test_125 failed with 2' in interop testing when a client whose test scripts still contain test_125 (2.16.x, and b2_15) runs against servers at 2.16.50.172 or later.

In the suite log, l_tunedisk leaves max_sectors_kb unchanged on the OST device:

Before: ost1 /dev/mapper/ost1_flakey 1279 2147483647
CMD: trevis-153vm34 l_tunedisk /dev/mapper/ost1_flakey
After: ost1 /dev/mapper/ost1_flakey 1279 2147483647
l_tunedisk failed to modify max_sectors_kb of ost1
 conf-sanity test_125: @@@@@@ FAIL: test_125 failed with 2

LU-12029 (https://review.whamcloud.com/57278, v2_16_50-172-g52dcb6d72e) removed max_sectors_kb tuning from l_tunedisk and deleted conf-sanity test_125 on master. The test still exists on b2_16 and b2_15, so it fails every time when those clients test against newer servers. The fix is a server version check in test_125 on the maintenance branches, skipping when OST1_VERSION >= 2.16.50.172 (or wherever LU-12029 landed), or backporting the test removal.

Example: lustre-master-next full-part-3, servers 2.17.59_78_g0c93230, client 2.16.1:
https://testing.whamcloud.com/test_sets/fad82d6c-0e43-4168-95c1-bd5053dba3c8

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