bcachefs: Fix an assertion in bch2_truncate()

We recently added an assertion that when we truncate a file to 0,
i_blocks should also go to 0 - but that's not necessarily true if we're
doing an emergency shutdown, lots of invariants no longer hold true in
that case.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/fs-io.c

@@ -2699,7 +2699,8 @@ int bch2_truncate(struct mnt_idmap *idmap,
 			U64_MAX, &i_sectors_delta);
 	i_sectors_acct(c, inode, NULL, i_sectors_delta);
 
-	BUG_ON(!inode->v.i_size && inode->v.i_blocks);
+	WARN_ON(!inode->v.i_size && inode->v.i_blocks &&
+		!bch2_journal_error(&c->journal));
 
 	if (unlikely(ret))
 		goto err;