bcachefs: Switch a BUG_ON() to a warning

This has popped and thus needs to be debugged, but the assertion firing
isn't necessarily fatal so switch it to a warning.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/journal.c

@@ -375,7 +375,8 @@ static int __journal_res_get(struct journal *j, struct journal_res *res,
 		goto retry;
 
 	if (ret == -ENOSPC) {
-		BUG_ON(!can_discard && (flags & JOURNAL_RES_GET_RESERVED));
+		WARN_ONCE(!can_discard && (flags & JOURNAL_RES_GET_RESERVED),
+			  "JOURNAL_RES_GET_RESERVED set but journal full");
 
 		/*
 		 * Journal is full - can't rely on reclaim from work item due to