Call zvol_create_minors() in spa_open_common() when initializing pool

There is an extremely odd bug that causes zvols to fail to appear on
some systems, but not others. Recently, I was able to consistently
reproduce this issue over a period of 1 month. The issue disappeared
after I applied this change from FreeBSD.

This is from FreeBSD's pool version 28 import, which occurred in
revision 219089.

Ported-by: Richard Yao <ryao@cs.stonybrook.edu>

module/zfs/spa.c

@@ -64,6 +64,7 @@
 #include <sys/zfs_ioctl.h>
 #include <sys/dsl_scan.h>
 #include <sys/zfeature.h>
+#include <sys/zvol.h>
 
 #ifdef	_KERNEL
 #include <sys/bootprops.h>
@@ -2856,6 +2857,7 @@ spa_open_common(const char *pool, spa_t **spapp, void *tag, nvlist_t *nvpolicy,
 	spa_load_state_t state = SPA_LOAD_OPEN;
 	int error;
 	int locked = B_FALSE;
+	int firstopen = B_FALSE;
 
 	*spapp = NULL;
 
@@ -2879,6 +2881,8 @@ spa_open_common(const char *pool, spa_t **spapp, void *tag, nvlist_t *nvpolicy,
 	if (spa->spa_state == POOL_STATE_UNINITIALIZED) {
 		zpool_rewind_policy_t policy;
 
+		firstopen = B_TRUE;
+
 		zpool_get_rewind_policy(nvpolicy ? nvpolicy : spa->spa_config,
 		    &policy);
 		if (policy.zrp_request & ZPOOL_DO_REWIND)
@@ -2953,6 +2957,11 @@ spa_open_common(const char *pool, spa_t **spapp, void *tag, nvlist_t *nvpolicy,
 		mutex_exit(&spa_namespace_lock);
 	}
 
+#ifdef _KERNEL
+	if (firstopen)
+		zvol_create_minors(spa->spa_name);
+#endif
+
 	*spapp = spa;
 
 	return (0);