f2fs: fix to do sanity check on segment bitmap of LFS curseg

f2fs: fix to do sanity check on segment bitmap of LFS curseg

As Jungyeon Reported in bugzilla:

https://bugzilla.kernel.org/show_bug.cgi?id=203233

- Reproduces
gcc poc_13.c
./run.sh f2fs

- Kernel messages
 F2FS-fs (sdb): Bitmap was wrongly set, blk:4608
 kernel BUG at fs/f2fs/segment.c:2133!
 RIP: 0010:update_sit_entry+0x35d/0x3e0
 Call Trace:
  f2fs_allocate_data_block+0x16c/0x5a0
  do_write_page+0x57/0x100
  f2fs_do_write_node_page+0x33/0xa0
  __write_node_page+0x270/0x4e0
  f2fs_sync_node_pages+0x5df/0x670
  f2fs_write_checkpoint+0x364/0x13a0
  f2fs_sync_fs+0xa3/0x130
  f2fs_do_sync_file+0x1a6/0x810
  do_fsync+0x33/0x60
  __x64_sys_fsync+0xb/0x10
  do_syscall_64+0x43/0x110
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

The testcase fails because that, in fuzzed image, current segment was
allocated with LFS type, its .next_blkoff should point to an unused
block address, but actually, its bitmap shows it's not. So during
allocation, f2fs crash when setting bitmap.

Introducing sanity_check_curseg() to check such inconsistence of
current in-used segment.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>