f2fs: fix to recover inode's i_gc_failures during POR

inode.i_gc_failures is used to indicate that skip count of migrating
on blocks of inode, we should guarantee it can be recovered in sudden
power-off case.

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