The dev_t argument of sys_mknod is passed to vfs_mknod, and is then cast
to int when foo_mknod is called, and is subsequently very often cast
back to dev_t.  (For example, minix_mknod() calls minix_set_inode() that
takes a dev_t.)

This is a cleanup that avoids this back-and-forth casting by giving
foo_mknod a prototype with dev_t.  In most cases now the dev_t is
transmitted untouched until init_special_inode.

It also makes the two routines hugetlbfs_get_inode() and
shmem_get_inode() static.