bcachefs: Fix btree_trans leak in bch2_readahead()

Reported-by: syzbot+d797fe78808e968d6c84@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-io-buffered.c

@@ -257,7 +257,6 @@ void bch2_readahead(struct readahead_control *ractl)
 	struct bch_inode_info *inode = to_bch_ei(ractl->mapping->host);
 	struct bch_fs *c = inode->v.i_sb->s_fs_info;
 	struct bch_io_opts opts;
-	struct btree_trans *trans = bch2_trans_get(c);
 	struct folio *folio;
 	struct readpages_iter readpages_iter;
 
@@ -269,6 +268,7 @@ void bch2_readahead(struct readahead_control *ractl)
 
 	bch2_pagecache_add_get(inode);
 
+	struct btree_trans *trans = bch2_trans_get(c);
 	while ((folio = readpage_iter_peek(&readpages_iter))) {
 		unsigned n = min_t(unsigned,
 				   readpages_iter.folios.nr -
@@ -289,10 +289,10 @@ void bch2_readahead(struct readahead_control *ractl)
 			   &readpages_iter);
 		bch2_trans_unlock(trans);
 	}
+	bch2_trans_put(trans);
 
 	bch2_pagecache_add_put(inode);
 
-	bch2_trans_put(trans);
 	darray_exit(&readpages_iter.folios);
 }