fs: yaffs2: replace CURRENT_TIME by other appropriate apis

The macro CURRENT_TIME has already been deleted by commit bfe1c566
("time: delete CURRENT_TIME_SEC and CURRENT_TIME"). So we need to
replace all the uses of CURRENT_TIME by current_time() for filesystem
times, and ktime_get_* function for others.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>