btrfs: fix WARNING in btrfs_select_ref_head()

btrfs: fix WARNING in btrfs_select_ref_head()

This issue was found when testing in-band dedupe enospc behaviour,
sometimes run_one_delayed_ref() may fail for enospc reason, then
__btrfs_run_delayed_refs（）will return, but forget to add num_heads_read
back, which will trigger "WARN_ON(delayed_refs->num_heads_ready == 0)" in
btrfs_select_ref_head().

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>
(cherry picked from commit 885ba6b0f5ddaf3e9c2da2afd96f3e5ff3c90ece)
(cherry picked from commit 9588cc0cb28c91a17677f40699423d97684e1f98)