bcachefs: If we run merges at a lower watermark, they must be nonblocking

Fix another deadlock related to the merge path; previously, we switched
to always running merges at a lower watermark (because they are
noncritical); but when we run at a lower watermark we also need to run
nonblocking or we've introduced a new deadlock.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-and-tested-by: s@m-h.ug

fs/bcachefs/btree_update_interior.c

@@ -1960,7 +1960,11 @@ int __bch2_foreground_maybe_merge(struct btree_trans *trans,
 	if ((flags & BCH_WATERMARK_MASK) == BCH_WATERMARK_interior_updates)
 		return 0;
 
-	flags &= ~BCH_WATERMARK_MASK;
+	if ((flags & BCH_WATERMARK_MASK) <= BCH_WATERMARK_reclaim) {
+		flags &= ~BCH_WATERMARK_MASK;
+		flags |= BCH_WATERMARK_btree;
+		flags |= BCH_TRANS_COMMIT_journal_reclaim;
+	}
 
 	b = trans->paths[path].l[level].b;