ext4: fix memory leak in parse_apply_sb_mount_options()

If processing the on-disk mount options fails after any memory was
allocated in the ext4_fs_context, e.g. s_qf_names, then this memory is
leaked.  Fix this by calling ext4_fc_free() instead of kfree() directly.

Reproducer:

    mkfs.ext4 -F /dev/vdc
    tune2fs /dev/vdc -E mount_opts=usrjquota=file
    echo clear > /sys/kernel/debug/kmemleak
    mount /dev/vdc /vdc
    echo scan > /sys/kernel/debug/kmemleak
    sleep 5
    echo scan > /sys/kernel/debug/kmemleak
    cat /sys/kernel/debug/kmemleak

Fixes: 7edfd85 ("ext4: Completely separate options parsing and sb setup")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Tested-by: Ritesh Harjani <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20220513231605.175121-2-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: ebiggers

fs/ext4/super.c

@@ -2628,8 +2628,10 @@ static int parse_apply_sb_mount_options(struct super_block *sb,
 	ret = ext4_apply_options(fc, sb);
 
 out_free:
-	kfree(s_ctx);
-	kfree(fc);
+	if (fc) {
+		ext4_fc_free(fc);
+		kfree(fc);
+	}
 	kfree(s_mount_opts);
 	return ret;
 }