xfs_da_grow_inode and xfs_dir2_grow_inode are mostly duplicate code.  Factor
the meat of those two functions into a new common helper.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NAlex Elder <aelder@sgi.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>