move mount_capable() calls to vfs_get_tree()

sget_fc() is called only from ->get_tree() instances and
the only instance not calling it is legacy_get_tree(),
which calls mount_capable() directly.

In all sget_fc() callers the checks could be moved to the
very beginning of ->get_tree() - ->user_ns is not changed
in between.  So lifting the checks to the only caller of
->get_tree() is OK.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fs_context.c

@@ -662,11 +662,6 @@ static int legacy_get_tree(struct fs_context *fc)
 	struct super_block *sb;
 	struct dentry *root;
 
-	if (!(fc->sb_flags & (SB_KERNMOUNT|SB_SUBMOUNT))) {
-		if (!mount_capable(fc))
-			return -EPERM;
-	}
-
 	root = fc->fs_type->mount(fc->fs_type, fc->sb_flags,
 				      fc->source, ctx->legacy_data);
 	if (IS_ERR(root))

fs/super.c

@@ -514,12 +514,6 @@ struct super_block *sget_fc(struct fs_context *fc,
 	struct user_namespace *user_ns = fc->global ? &init_user_ns : fc->user_ns;
 	int err;
 
-	if (!(fc->sb_flags & SB_KERNMOUNT) &&
-	    fc->purpose != FS_CONTEXT_FOR_SUBMOUNT) {
-		if (!mount_capable(fc))
-			return ERR_PTR(-EPERM);
-	}
-
 retry:
 	spin_lock(&sb_lock);
 	if (test) {
@@ -1421,6 +1415,12 @@ 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.
 	 */