teach vfs_get_tree() to handle subtype, switch do_new_mount() to it

Roll the handling of subtypes into do_new_mount() and vfs_get_tree().  The
former determines any subtype string and hangs it off the fs_context; the
latter applies it.

Make do_new_mount() create, parameterise and commit an fs_context and
create a mount for itself rather than calling vfs_kern_mount().

[AV -- missing kstrdup()]
[AV -- ... and no kstrdup() if we get to setting ->s_submount - we
simply transfer it from fc, leaving NULL behind]
[AV -- constify ->s_submount, while we are at it]

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -1447,7 +1447,7 @@ struct super_block {
 	 * Filesystem subtype.  If non-empty the filesystem type field
 	 * in /proc/mounts will be "type.subtype"
 	 */
-	char *s_subtype;
+	const char *s_subtype;
 
 	const struct dentry_operations *s_d_op; /* default d_op for dentries */