dm writecache: skip writecache_wait when using pmem mode

The array bio_in_progress is only used with ssd mode. So skip
writecache_wait_for_ios in writecache_discard when pmem mode.
Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -849,8 +849,10 @@ static void writecache_discard(struct dm_writecache *wc, sector_t start, sector_
 
 		if (likely(!e->write_in_progress)) {
 			if (!discarded_something) {
+				if (!WC_MODE_PMEM(wc)) {
 					writecache_wait_for_ios(wc, READ);
 					writecache_wait_for_ios(wc, WRITE);
+				}
 				discarded_something = true;
 			}
 			if (!writecache_entry_is_committed(wc, e))