nfsd: use ktime_get_real_seconds() in nfs4_verifier

gen_confirm() generates a unique identifier based on the current
time. This overflows in year 2038, but that is harmless since it
generally does not lead to duplicates, as long as the time has
been initialized by a real-time clock or NTP.

Using ktime_get_boottime_seconds() or ktime_get_seconds() would
avoid the overflow, but it would be more likely to result in
non-unique numbers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>