tracefs: Set the group ownership in apply_options() not parse_options()

Al Viro brought it to my attention that the dentries may not be filled
when the parse_options() is called, causing the call to set_gid() to
possibly crash. It should only be called if parse_options() succeeds
totally anyway.

He suggested the logical place to do the update is in apply_options().

Link: https://lore.kernel.org/all/20220225165219.737025658@goodmis.org/
Link: https://lkml.kernel.org/r/20220225153426.1c4cab6b@gandalf.local.home

Cc: stable@vger.kernel.org
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Fixes: 48b27b6 ("tracefs: Set all files to the same group ownership as the mount option")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/inode.c

@@ -264,7 +264,6 @@ static int tracefs_parse_options(char *data, struct tracefs_mount_opts *opts)
 			if (!gid_valid(gid))
 				return -EINVAL;
 			opts->gid = gid;
-			set_gid(tracefs_mount->mnt_root, gid);
 			break;
 		case Opt_mode:
 			if (match_octal(&args[0], &option))
@@ -291,7 +290,9 @@ static int tracefs_apply_options(struct super_block *sb)
 	inode->i_mode |= opts->mode;
 
 	inode->i_uid = opts->uid;
-	inode->i_gid = opts->gid;
+
+	/* Set all the group ids to the mount option */
+	set_gid(sb->s_root, opts->gid);
 
 	return 0;
 }