init: add an init_mount helper

Like do_mount, but takes a kernel pointer for the destination path.
Switch over the mounts in the init code and devtmpfs to it, which
just happen to work due to the implicit set_fs(KERNEL_DS) during early
init right now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/internal.h b/fs/internal.h
index e903d5a..72ea0b6 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -89,6 +89,10 @@ extern int __mnt_want_write_file(struct file *);
 extern void __mnt_drop_write_file(struct file *);
 
 extern void dissolve_on_fput(struct vfsmount *);
+
+int path_mount(const char *dev_name, struct path *path,
+		const char *type_page, unsigned long flags, void *data_page);
+
 /*
  * fs_struct.c
  */