f2fs: replace F2FS_I(inode) and sbi by the local variable

We have define 'fi' at the begin of the functions, just use it,
rather than use F2FS_I(inode) again.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
[Jaegeuk Kim: replace sbi]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>