xfs: don't hold xattr leaf buffers across transaction rolls

Now that we've established (again!) that empty xattr leaf buffers are
ok, we no longer need to bhold them to transactions when we're creating
new leaf blocks.  Get rid of the entire mechanism, which should simplify
the xattr code quite a bit.

The original justification for using bhold here was to prevent the AIL
from trying to write the empty leaf block into the fs during the brief
time that we release the buffer lock.  The reason for /that/ was to
prevent recovery from tripping over the empty ondisk block.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>