f2fs: don't access node/meta inode mapping after iput

This fixes wrong access of address spaces of node and meta inodes after iput.

Fixes: 60aa4d55

 ("f2fs: fix use-after-free issue when accessing sbi->stat_info")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>