erofs: refuse crafted out-of-file-range encoded extents

Crafted encoded extents could record out-of-range `lstart`, which should
not happen in normal cases.

It caused an iomap_iter_done() complaint [1] reported by syzbot.

[1] https://lore.kernel.org/r/684cb499.a00a0220.c6bd7.0010.GAE@google.com

Fixes: 1d191b4ca5 ("erofs: implement encoded extent metadata")
Reported-and-tested-by: syzbot+d8f000c609f05f52d9b5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d8f000c609f05f52d9b5
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250619032839.2642193-1-hsiangkao@linux.alibaba.com

fs/erofs/zmap.c

@@ -597,6 +597,10 @@ static int z_erofs_map_blocks_ext(struct inode *inode,
 
 			if (la > map->m_la) {
 				r = mid;
+				if (la > lend) {
+					DBG_BUGON(1);
+					return -EFSCORRUPTED;
+				}
 				lend = la;
 			} else {
 				l = mid + 1;