nfds: avoid gettimeofday for nfssvc_boot time

do_gettimeofday() is deprecated and we should generally use time64_t
based functions instead.

In case of nfsd, all three users of nfssvc_boot only use the initial
time as a unique token, and are not affected by it overflowing, so they
are not affected by the y2038 overflow.

This converts the structure to timespec64 anyway and adds comments
to all uses, to document that we have thought about it and avoid
having to look at it again.

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