[XFS] Fix mknod regression

[XFS] Fix mknod regression

This was broken by my '[XFS] simplify xfs_create/mknod/symlink prototype',
which assigned the re-shuffled ondisk dev_t back to the rdev variable in
xfs_vn_mknod. Because of that i_rdev is set to the ondisk dev_t instead of
the linux dev_t later down the function.

Fortunately the fix for it is trivial: we can just remove the assignment
because xfs_revalidate_inode has done the proper job before unlocking the
inode.

SGI-PV: 974873
SGI-Modid: xfs-linux-melb:xfs-kern:30273a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>