This is work spun out of https://review.whamcloud.com/#/c/34058/ / LU-11868.

The sizing used for the various RPC fields in the MDC code is a bit chaotic.

Some of them use cl_max_mds_easize, some use cl_default_mds_easize, and some use ocd_max_easize, which is the raw easize from the server, and without LU-11868, can be as much as 1 MiB.

We need to look over the code setting up the repbuf for the various operations and decide on:

1. Reasonable defaults (the ACL buffer is currently set to max_easize, which is much too large for a default for ACLs)
2. Good behavior for handling large layouts or other eas - The current code is inconsistent, but the intended behavior of default_mds_easize is that it expands with the sizes seen up to a limit of 4K...  But it never drops. 

The current behavior is workable and LU-11868 improves it, but there is definitely some technical debt here.