f2fs: not allow to write illegal blkaddr

we came across an error as below:

[build_nat_area_bitmap:1710] nid[0x    1718] addr[0x         1c18ddc] ino[0x    1718]
[build_nat_area_bitmap:1710] nid[0x    1719] addr[0x         1c193d5] ino[0x    1719]
[build_nat_area_bitmap:1710] nid[0x    171a] addr[0x         1c1736e] ino[0x    171a]
[build_nat_area_bitmap:1710] nid[0x    171b] addr[0x        58b3ee8f] ino[0x815f92ed]
[build_nat_area_bitmap:1710] nid[0x    171c] addr[0x         fcdc94b] ino[0x49366377]
[build_nat_area_bitmap:1710] nid[0x    171d] addr[0x        7cd2facf] ino[0xb3c55300]
[build_nat_area_bitmap:1710] nid[0x    171e] addr[0x        bd4e25d0] ino[0x77c34c09]

... ...

[build_nat_area_bitmap:1710] nid[0x    1718] addr[0x         1c18ddc] ino[0x    1718]
[build_nat_area_bitmap:1710] nid[0x    1719] addr[0x         1c193d5] ino[0x    1719]
[build_nat_area_bitmap:1710] nid[0x    171a] addr[0x         1c1736e] ino[0x    171a]
[build_nat_area_bitmap:1710] nid[0x    171b] addr[0x        58b3ee8f] ino[0x815f92ed]
[build_nat_area_bitmap:1710] nid[0x    171c] addr[0x         fcdc94b] ino[0x49366377]
[build_nat_area_bitmap:1710] nid[0x    171d] addr[0x        7cd2facf] ino[0xb3c55300]
[build_nat_area_bitmap:1710] nid[0x    171e] addr[0x        bd4e25d0] ino[0x77c34c09]

One nat block may be stepped by a data block, so this patch forbid to
write if the blkaddr is illegal

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>