btrfs: do not shorten unpin len for caching block groups

While fixing up our ->last_byte_to_unpin locking I noticed that we will
shorten len based on ->last_byte_to_unpin if we're caching when we're
adding back the free space.  This is correct for the free space, as we
cannot unpin more than ->last_byte_to_unpin, however we use len to
adjust the ->bytes_pinned counters and such, which need to track the
actual pinned usage.  This could result in
WARN_ON(space_info->bytes_pinned) triggering at unmount time.

Fix this by using a local variable for the amount to add to free space
cache, and leave len untouched in this case.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -2794,10 +2794,10 @@ static int unpin_extent_range(struct btrfs_fs_info *fs_info,
 		len = cache->start + cache->length - start;
 		len = min(len, end + 1 - start);
 
-		if (start < cache->last_byte_to_unpin) {
-			len = min(len, cache->last_byte_to_unpin - start);
-			if (return_free_space)
-				btrfs_add_free_space(cache, start, len);
+		if (start < cache->last_byte_to_unpin && return_free_space) {
+			u64 add_len = min(len, cache->last_byte_to_unpin - start);
+
+			btrfs_add_free_space(cache, start, add_len);
 		}
 
 		start += len;