The only caller (xfs_fs_fill_super) can simplify call igrab on the root
inode.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:30393a
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>