stex: Remove use of struct timeval

Function stex_gettime uses 'struct timeval' whose tv_sec value
will overflow on 32-bit systems in year 2038 and beyond. This patch
replaces the use of struct timeval and do_gettimeofday with
ktime_get_real_seconds, which returns a 64-bit seconds value.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tina Ruchandani <ruchandani.tina@gmail.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>