f2fs: fix to do sanity check on node blkaddr in truncate_node()

f2fs: fix to do sanity check on node blkaddr in truncate_node()

syzbot reports a f2fs bug as below:

------------[ cut here ]------------
kernel BUG at fs/f2fs/segment.c:2534!
RIP: 0010:f2fs_invalidate_blocks+0x35f/0x370 fs/f2fs/segment.c:2534
Call Trace:
 truncate_node+0x1ae/0x8c0 fs/f2fs/node.c:909
 f2fs_remove_inode_page+0x5c2/0x870 fs/f2fs/node.c:1288
 f2fs_evict_inode+0x879/0x15c0 fs/f2fs/inode.c:856
 evict+0x4e8/0x9b0 fs/inode.c:723
 f2fs_handle_failed_inode+0x271/0x2e0 fs/f2fs/inode.c:986
 f2fs_create+0x357/0x530 fs/f2fs/namei.c:394
 lookup_open fs/namei.c:3595 [inline]
 open_last_lookups fs/namei.c:3694 [inline]
 path_openat+0x1c03/0x3590 fs/namei.c:3930
 do_filp_open+0x235/0x490 fs/namei.c:3960
 do_sys_openat2+0x13e/0x1d0 fs/open.c:1415
 do_sys_open fs/open.c:1430 [inline]
 __do_sys_openat fs/open.c:1446 [inline]
 __se_sys_openat fs/open.c:1441 [inline]
 __x64_sys_openat+0x247/0x2a0 fs/open.c:1441
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0010:f2fs_invalidate_blocks+0x35f/0x370 fs/f2fs/segment.c:2534

The root cause is: on a fuzzed image, blkaddr in nat entry may be
corrupted, then it will cause system panic when using it in
f2fs_invalidate_blocks(), to avoid this, let's add sanity check on
nat blkaddr in truncate_node().

Reported-by: syzbot+33379ce4ac76acf7d0c7@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-f2fs-devel/0000000000009a6cd706224ca720@google.com/
Cc: stable@vger.kernel.org
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>