LU-11868 mdc: Improve xattr buffer allocations

LU-11868 mdc: Improve xattr buffer allocations

Many of the xattr related buffers in the mdc/mdt code are
allocated at max_easize, but they are used for normal POSIX
xattrs (primarily ACLs) and so they are guaranteed not to
exceed XATTR_SIZE_MAX.

HSM xattrs should also be less than XATTR_SIZE_MAX.

Reduce allocations to MIN(XATTR_SIZE_MAX, max_easize).

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I228486ebd79b35358baeff490d6181cefa5a4586
Reviewed-on: https://review.whamcloud.com/34059
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>