fsck.f2fs: check validity of nat journal

fsck.f2fs: check validity of nat journal

As reported by Aravind:

I built f2fs tools from source (at tag v1.12.0) and was able to get this backtrace in gdb:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7f8eb54 in f2fs_set_bit (nr=1041170432,
    addr=0x7fffff621010 <error: Cannot access memory at address 0x7fffff621010>) at libf2fs.c:312
312        mask = 1 << (7 - (nr & 0x07));
(gdb) where
    addr=0x7fffff621010 <error: Cannot access memory at address 0x7fffff621010>) at libf2fs.c:312

> [ 5338.040024] nats:8781, sits:6
> [ 5338.040027] F2FS-fs (sda2): Failed to initialize F2FS segment manager
> [ 5338.128893] nats:8781, sits:6
> [ 5338.128895] F2FS-fs (sda2): Failed to initialize F2FS segment manager

nat_count/nid/blkaddr recorded in journal may be corrupted, let's do
sanity check on them, skip loading invalid ones during build_node_manager().

Reported-by: Aravind R S <aravindet@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>