Linux 5.6 compat: time_t

As part of the Linux kernel's y2038 changes the time_t type has been
fully retired.  Callers are now required to use the time64_t type.

Rather than move to the new type, I've removed the few remaining
places where a time_t is used in the kernel code.  They've been
replaced with a uint64_t which is already how ZFS internally
handled these values.

Going forward we should work towards updating the remaining user
space time_t consumers to the 64-bit interfaces.

Reviewed-by: Matthew Macy <mmacy@freebsd.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10052
Closes #10064

lib/libspl/include/sys/time.h

@@ -88,7 +88,7 @@ gethrestime(inode_timespec_t *ts)
 	ts->tv_nsec = tv.tv_usec * NSEC_PER_USEC;
 }
 
-static inline time_t
+static inline uint64_t
 gethrestime_sec(void)
 {
 	struct timeval tv;