Let zpool import ignore a missing hostid record.

Change the zpool program to skip its hostid mismatch check in the same way that
libzfs already does.

Invoked imports fail if the ZPOOL_CONFIG_HOSTID nvpair is missing in the
/etc/zfs/zpool.cache file, which can happen as of the /etc/hostid deprecation
in commit openzfs/spl@acf0ade.

Closes: openzfs#2794

cmd/zpool/zpool_main.c

@@ -1914,37 +1914,30 @@ do_import(nvlist_t *config, const char *newname, const char *mntopts,
 		return (1);
 	} else if (state != POOL_STATE_EXPORTED &&
 	    !(flags & ZFS_IMPORT_ANY_HOST)) {
-		uint64_t hostid;
-
-		if (nvlist_lookup_uint64(config, ZPOOL_CONFIG_HOSTID,
-		    &hostid) == 0) {
-			unsigned long system_hostid = gethostid() & 0xffffffff;
-
-			if ((unsigned long)hostid != system_hostid) {
-				char *hostname;
-				uint64_t timestamp;
-				time_t t;
-
-				verify(nvlist_lookup_string(config,
-				    ZPOOL_CONFIG_HOSTNAME, &hostname) == 0);
-				verify(nvlist_lookup_uint64(config,
-				    ZPOOL_CONFIG_TIMESTAMP, &timestamp) == 0);
-				t = timestamp;
-				(void) fprintf(stderr, gettext("cannot import "
-				    "'%s': pool may be in use from other "
-				    "system, it was last accessed by %s "
-				    "(hostid: 0x%lx) on %s"), name, hostname,
-				    (unsigned long)hostid,
-				    asctime(localtime(&t)));
-				(void) fprintf(stderr, gettext("use '-f' to "
-				    "import anyway\n"));
-				return (1);
-			}
-		} else {
-			(void) fprintf(stderr, gettext("cannot import '%s': "
-			    "pool may be in use from other system\n"), name);
-			(void) fprintf(stderr, gettext("use '-f' to import "
-			    "anyway\n"));
+		uint64_t hostid = 0;
+		unsigned long system_hostid = gethostid() & 0xffffffff;
+
+		(void) nvlist_lookup_uint64(config, ZPOOL_CONFIG_HOSTID,
+		    &hostid);
+
+		if (hostid != 0 && (unsigned long)hostid != system_hostid) {
+			char *hostname;
+			uint64_t timestamp;
+			time_t t;
+
+			verify(nvlist_lookup_string(config,
+			    ZPOOL_CONFIG_HOSTNAME, &hostname) == 0);
+			verify(nvlist_lookup_uint64(config,
+			    ZPOOL_CONFIG_TIMESTAMP, &timestamp) == 0);
+			t = timestamp;
+			(void) fprintf(stderr, gettext("cannot import "
+			    "'%s': pool may be in use from other "
+			    "system, it was last accessed by %s "
+			    "(hostid: 0x%lx) on %s"), name, hostname,
+			    (unsigned long)hostid,
+			    asctime(localtime(&t)));
+			(void) fprintf(stderr, gettext("use '-f' to "
+			    "import anyway\n"));
 			return (1);
 		}
 	}