Pack zrlock_t by 8 bytes

On FreeBSD this reduces this structure size from 64 to 56 bytes.
dnode_handle_t respectively reduces from 72 to 64 bytes. It sounds
like a waste to need 72 bytes to be able to relocate 808 bytes of
dnode_t, which relocation on FreeBSD is not even supported.

Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Richard Yao <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by:  Alexander Motin <[email protected]>
Sponsored by:   iXsystems, Inc.
Closes #14317

include/sys/zrlock.h

@@ -34,9 +34,8 @@ extern "C" {
 
 typedef struct zrlock {
 	kmutex_t zr_mtx;
-	volatile int32_t zr_refcount;
 	kcondvar_t zr_cv;
-	uint16_t zr_pad;
+	volatile int32_t zr_refcount;
 #ifdef	ZFS_DEBUG
 	kthread_t *zr_owner;
 	const char *zr_caller;