LU-12784 llite: limit max xattr size by kernel value

LU-12784 llite: limit max xattr size by kernel value

Limit the maximum xattr size returned to userspace from the MDS to
what the currently-running kernel supports (XATTR_SIZE_MAX=65536
bytes typically).  While it is possible a Lustre backing filesystem
may store larger xattrs than this, it wouldn't be possible for users
to access a larger xattr via kernel xattr interfaces.

This fixes interop problems when newer clients and tests are running
against older servers:

  sanity.sh: line 8946: /usr/bin/setfattr: Argument list too long

Skip subtests for new features in 2.13 so 2.12 interop testing passes.

Fix test-framework.sh::large_xattr_enabled() to return true for ZFS.
Fix test-framework.sh::max_xattr_size() to return the actual value
returned from the MDS rather than computing it locally.

Fixes: 3ec712bd183 ("LU-11868 osd: Set max ea size to XATTR_SIZE_MAX")
Test-Parameters: trivial serverversion=2.12 testlist=sanity
Test-Parameters: serverversion=2.12 testlist=conf-sanity envdefinitions=ONLY=81
Test-Parameters: testlist=sanity-pfl,replay-single
Test-Parameters: testlist=conf-sanity envdefinitions=ONLY=48,61,81
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I14232809b13886efa8f11a50ecc35e78f316810d
Reviewed-on: https://review.whamcloud.com/36240
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>