xfs: factor out xfs_da_grow_inode_int

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: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>