nfsd: use ktime_get_seconds() for timestamps

[ Upstream commit b3f255ef ]

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: 45bb63ed ("nfsd: fix delegation_blocked() to block correctly for at least 30 seconds")
Signed-off-by: Sasha Levin <sashal@kernel.org>