f2fs: avoid using timespec

All uses of timespec are deprecated, and this one is not particularly
useful, as the documented method for converting seconds to jiffies
is to multiply by 'HZ'.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>