f2fs: add static init_idisk_time function to reduce the code

We can use a inner function to init the disk time
of f2fs_inode_info for cleaning redundant code.

Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>