f2fs: fix to detect inconsistent nat entry during truncation

As Roman Smirnov reported as below:

"
There is a possible bug in f2fs_truncate_inode_blocks():

    if (err < 0 && err != -ENOENT)
    			goto fail;
        ...
        offset[1] = 0;
        offset[0]++;
        nofs += err;

If err = -ENOENT then nofs will sum with an error code,
which is strange behaviour. Also if nofs < ENOENT this will
cause an overflow. err will be equal to -ENOENT with the
following call stack:

truncate_nodes()
  f2fs_get_node_page()
     __get_node_page()
        read_node_page()
"

If nat is corrupted, truncate_nodes() may return -ENOENT, and
f2fs_truncate_inode_blocks() doesn't handle such error correctly,
fix it.

Reported-by: Roman Smirnov <r.smirnov@omp.ru>
Closes: https://lore.kernel.org/linux-f2fs-devel/085b27fd2b364a3c8c3a9ca77363e246@omp.ru


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