fsck.f2fs: report real wall time

fsck.f2fs: report real wall time

clock_t time is per-process time, which is not wall time. For example,
if the CPU is shared by other processes, clock_t time may advance slower
than wall clock. On the other hand, if the current process is
multithreaded and more than one execution core is available, clock_t
time may advance faster than wall clock.

this CL changes it to use CLOCK_BOOTTIME (Linux-specific)

Signed-off-by: Wei Wang <wvw@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>