Avoid a gcc -Wint-to-pointer-cast warning

On 32-bit platforms long long is generally 64-bits.  Sufficiently modern
versions of gcc (13 in my testing) complains when casting a pointer to
an integer of a different width so cast to uintptr_t first to avoid the
warning.

Fixes: c183d16 Parallel pool import

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Don Brady <don.brady@klarasystems.com>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes openzfs#16203

module/zfs/spa.c

@@ -6827,7 +6827,7 @@ spa_tryimport(nvlist_t *tryconfig)
 	 */
 	char *name = kmem_alloc(MAXPATHLEN, KM_SLEEP);
 	(void) snprintf(name, MAXPATHLEN, "%s-%llx-%s",
-	    TRYIMPORT_NAME, (u_longlong_t)curthread, poolname);
+	    TRYIMPORT_NAME, (u_longlong_t)(uintptr_t)curthread, poolname);
 
 	mutex_enter(&spa_namespace_lock);
 	spa = spa_add(name, tryconfig, NULL);