Fix static data to link with -fno-common

-fno-common is the new default in GCC 10, replacing -fcommon in
GCC <= 9, so static data must only be allocated once.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Romain Dolbeau <romain.dolbeau@european-processor-initiative.eu>
Closes #9943

cmd/zfs/zfs_util.h

@@ -33,7 +33,7 @@ extern "C" {
 
 void * safe_malloc(size_t size);
 void nomem(void);
-libzfs_handle_t *g_zfs;
+extern libzfs_handle_t *g_zfs;
 
 #ifdef	__cplusplus
 }

cmd/zpool/zpool_main.c

@@ -73,6 +73,8 @@
 
 #include "statcommon.h"
 
+libzfs_handle_t *g_zfs;
+
 static int zpool_do_create(int, char **);
 static int zpool_do_destroy(int, char **);
 

cmd/zpool/zpool_util.h

@@ -80,7 +80,7 @@ void pool_list_free(zpool_list_t *);
 int pool_list_count(zpool_list_t *);
 void pool_list_remove(zpool_list_t *, zpool_handle_t *);
 
-libzfs_handle_t *g_zfs;
+extern libzfs_handle_t *g_zfs;
 
 
 typedef	struct vdev_cmd_data

lib/libshare/smb.c

@@ -65,6 +65,8 @@ static boolean_t smb_available(void);
 
 static sa_fstype_t *smb_fstype;
 
+smb_share_t *smb_shares;
+
 /*
  * Retrieve the list of SMB shares.
  */

lib/libshare/smb.h

@@ -44,6 +44,6 @@ typedef struct smb_share_s {
 	struct smb_share_s *next;
 } smb_share_t;
 
-smb_share_t *smb_shares;
+extern smb_share_t *smb_shares;
 
 void libshare_smb_init(void);