dm writecache: skip writecache_wait for pmem mode

The array bio_in_progress[2] only have chance to be increased and
decreased with ssd mode. For pmem mode, they are not involved at all.
So skip writecache_wait_for_ios in writecache_flush for pmem.

Suggested-by: Doris Yu <[e-mail]>
Signed-off-by: Huaisheng Ye <[e-mail]>
Acked-by: Mikulas Patocka <[e-mail]>
Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-writecache.c

···
 	}
 	writecache_commit_flushed(wc);
 
-	writecache_wait_for_ios(wc, WRITE);
+	if (!WC_MODE_PMEM(wc))
+		writecache_wait_for_ios(wc, WRITE);
 
 	wc->seq_count++;
 	pmem_assign(sb(wc)->seq_count, cpu_to_le64(wc->seq_count));