ext4: protect bb_first_free in ext4_trim_all_free() with group lock

We should protect reading bd_info->bb_first_free with the group lock
because otherwise we might miss some free blocks. This is not a big deal
at all, but the change to do right thing is really simple, so lets do
that.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -4825,11 +4825,11 @@ ext4_trim_all_free(struct super_block *sb, ext4_group_t group,
 				"information for %u", group);
 		return ret;
 	}
-
 	bitmap = e4b.bd_bitmap;
+
+	ext4_lock_group(sb, group);
 	start = (e4b.bd_info->bb_first_free > start) ?
 		e4b.bd_info->bb_first_free : start;
-	ext4_lock_group(sb, group);
 
 	while (start < max) {
 		start = mb_find_next_zero_bit(bitmap, max, start);