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

module/os/linux/zfs/zfs_debug.c

@@ -28,7 +28,7 @@
 
 typedef struct zfs_dbgmsg {
 	procfs_list_node_t	zdm_node;
-	time_t			zdm_timestamp;
+	uint64_t		zdm_timestamp;
 	int			zdm_size;
 	char			zdm_msg[1]; /* variable length allocation */
 } zfs_dbgmsg_t;