Avoid possibility of division by zero

When hz > 1000, msec / (1000 / hz) results in division by zero.

I found somewhere in FreeBSD using howmany(msec * hz, 1000) to convert
ms to ticks, avoiding the potential for a zero in the divisor.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10894

include/os/freebsd/zfs/sys/zfs_context_os.h

@@ -72,7 +72,7 @@ extern struct mtx zfs_debug_mtx;
 	}	   \
 } while (0)
 
-#define	MSEC_TO_TICK(msec)	((msec) / (MILLISEC / hz))
+#define	MSEC_TO_TICK(msec)	(howmany((hrtime_t)(msec) * hz, MILLISEC))
 extern int hz;
 extern int tick;
 typedef int fstrans_cookie_t;

include/sys/zfs_context.h

@@ -602,9 +602,9 @@ typedef struct vsecattr {
 extern void delay(clock_t ticks);
 
 #define	SEC_TO_TICK(sec)	((sec) * hz)
-#define	MSEC_TO_TICK(msec)	((msec) / (MILLISEC / hz))
-#define	USEC_TO_TICK(usec)	((usec) / (MICROSEC / hz))
-#define	NSEC_TO_TICK(usec)	((usec) / (NANOSEC / hz))
+#define	MSEC_TO_TICK(msec)	(howmany((hrtime_t)(msec) * hz, MILLISEC))
+#define	USEC_TO_TICK(usec)	(howmany((hrtime_t)(usec) * hz, MICROSEC))
+#define	NSEC_TO_TICK(nsec)	(howmany((hrtime_t)(nsec) * hz, NANOSEC))
 
 #define	max_ncpus	64
 #define	boot_ncpus	(sysconf(_SC_NPROCESSORS_ONLN))