[XFS] simplify validata_fields

Stop using xfs_getattr and a onstack bhv_vattr_t just to get three fields
from the underlying inode and opencode copying from the inode fields
instead.

SGI-PV: 970662
SGI-Modid: xfs-linux-melb:xfs-kern:29711a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>