spa: avoid type narrowing warning

Building the spa module for i386 caused gcc to emit
-Wint-to-pointer-cast "cast to pointer from integer of different size"
because spa.spa_did was uint64_t but pthread_join (via thread_join in
spa_deactivate) takes a pointer (32-bit on i386).  Define spa_did to be
pointer-size instead.  For now spa_did is in fact never non-zero and the
thread_join could instead be ifdef'd out, but changing the size of
spa_did may be more useful for the future.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Libby <rlibby@FreeBSD.org>
Closes #11336

include/sys/spa_impl.h

@@ -378,7 +378,7 @@ struct spa {
 	kcondvar_t	spa_proc_cv;		/* spa_proc_state transitions */
 	spa_proc_state_t spa_proc_state;	/* see definition */
 	proc_t		*spa_proc;		/* "zpool-poolname" process */
-	uint64_t	spa_did;		/* if procp != p0, did of t1 */
+	uintptr_t	spa_did;		/* if procp != p0, did of t1 */
 	boolean_t	spa_autoreplace;	/* autoreplace set in open */
 	int		spa_vdev_locks;		/* locks grabbed */
 	uint64_t	spa_creation_version;	/* version at pool creation */