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>

init/do_mounts.h

@@ -8,6 +8,7 @@
 #include <linux/mount.h>
 #include <linux/major.h>
 #include <linux/root_dev.h>
+#include <linux/init_syscalls.h>
 
 void  mount_block_root(char *name, int flags);
 void  mount_root(void);