xfs: delete unnecessary NULL checks

These NULL check are no long needed after commit 2ed5b09b ("xfs:
make inode attribute forks a permanent part of struct xfs_inode").

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>