move mount_capable() further out

Call graph of vfs_get_tree():
	vfs_fsconfig_locked()	# neither kernmount, nor submount
	do_new_mount()		# neither kernmount, nor submount
	fc_mount()
		afs_mntpt_do_automount()	# submount
		mount_one_hugetlbfs()		# kernmount
		pid_ns_prepare_proc()		# kernmount
		mq_create_mount()		# kernmount
		vfs_kern_mount()
			simple_pin_fs()		# kernmount
			vfs_submount()		# submount
			kern_mount()		# kernmount
			init_mount_tree()
			btrfs_mount()
			nfs_do_root_mount()

	The first two need the check (unconditionally).
init_mount_tree() is setting rootfs up; any capability
checks make zero sense for that one.  And btrfs_mount()/
nfs_do_root_mount() have the checks already done in their
callers.

	IOW, we can shift mount_capable() handling into
the two callers - one in the normal case of mount(2),
another - in fsconfig(2) handling of FSCONFIG_CMD_CREATE.
I.e. the syscalls that set a new filesystem up.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fsopen.c

@@ -230,6 +230,8 @@ static int vfs_fsconfig_locked(struct fs_context *fc, int cmd,
 	case FSCONFIG_CMD_CREATE:
 		if (fc->phase != FS_CONTEXT_CREATE_PARAMS)
 			return -EBUSY;
+		if (!mount_capable(fc))
+			return -EPERM;
 		fc->phase = FS_CONTEXT_CREATING;
 		ret = vfs_get_tree(fc);
 		if (ret)

fs/namespace.c

@@ -2786,6 +2786,8 @@ static int do_new_mount(struct path *path, const char *fstype, int sb_flags,
 		err = vfs_parse_fs_string(fc, "source", name, strlen(name));
 	if (!err)
 		err = parse_monolithic_mount_data(fc, data);
+	if (!err && !mount_capable(fc))
+		err = -EPERM;
 	if (!err)
 		err = vfs_get_tree(fc);
 	if (!err)

fs/super.c

@@ -1415,12 +1415,6 @@ int vfs_get_tree(struct fs_context *fc)
 	if (fc->root)
 		return -EBUSY;
 
-	if (!(fc->sb_flags & SB_KERNMOUNT) &&
-	    fc->purpose != FS_CONTEXT_FOR_SUBMOUNT) {
-		if (!mount_capable(fc))
-			return -EPERM;
-	}
-
 	/* Get the mountable root in fc->root, with a ref on the root and a ref
 	 * on the superblock.
 	 */