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

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);