LU-10513 acl: prepare small buffer for ACL RPC reply

LU-10513 acl: prepare small buffer for ACL RPC reply

For most of files, their ACL entries are very limited, under
such case, it is unnecessary to prepare very large reply buffer
to hold unknown-sized ACL entries for the getattr/open RPCs.
Instead, we can prepare some relative small buffer, such as the
LUSTRE_POSIX_ACL_MAX_SIZE_OLD (260) bytes, that is equal to the
ACL size before patch 64b2fad22a4eb4727315709e014d8f74c5a7f289.
If the target file has too many ACL entries and exceeds the
prepared reply buffer, then the MDT will reply -ERANGE failure
to the client, and then the client can prepare more large buffer
and try again. Since the file with large ACL is rare case, such
retrying getattr/open RPCs will not affect the real performance
too much.

The advantage is that it reduces the client side RAM pressure.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I4c01b19520cab1cc712e36f3b0225973fba00410
Reviewed-on: https://review.whamcloud.com/28116
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>