nfsd: use ktime_get_seconds() for timestamps

[ Upstream commit b3f255ef6bffc18a28c3b6295357f2a3380c033f ]

The delegation logic in nfsd uses the somewhat inefficient
seconds_since_boot() function to record time intervals.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Stable-dep-of: 45bb63ed20e0 ("nfsd: fix delegation_blocked() to block correctly for at least 30 seconds")
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit f81fcf39509d30cb5f1c659099c1d8f0c2a9a57a)
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>