bcachefs: fiemap: Fix a lockdep splat

As with the previous patch, we generally can't hold btree locks while
copying to userspace, as that may incur a page fault and require
mmap_lock.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs.c

@@ -943,6 +943,7 @@ static int bch2_fiemap(struct inode *vinode, struct fiemap_extent_info *info,
 		cur.k->k.p.offset += cur.k->k.size;
 
 		if (have_extent) {
+			bch2_trans_unlock(&trans);
 			ret = bch2_fill_extent(c, info,
 					bkey_i_to_s_c(prev.k), 0);
 			if (ret)
@@ -961,9 +962,11 @@ static int bch2_fiemap(struct inode *vinode, struct fiemap_extent_info *info,
 	if (bch2_err_matches(ret, BCH_ERR_transaction_restart))
 		goto retry;
 
-	if (!ret && have_extent)
+	if (!ret && have_extent) {
+		bch2_trans_unlock(&trans);
 		ret = bch2_fill_extent(c, info, bkey_i_to_s_c(prev.k),
 				       FIEMAP_EXTENT_LAST);
+	}
 
 	bch2_trans_exit(&trans);
 	bch2_bkey_buf_exit(&cur, c);