libshare: nfs: open temporary file once

Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12067

lib/libshare/os/linux/smb.c

@@ -254,7 +254,7 @@ smb_enable_share_one(const char *sharename, const char *sharepath)
 	argv[5] = (char *)name;
 	argv[6] = (char *)sharepath;
 	argv[7] = (char *)comment;
-	argv[8] = "Everyone:F";
+	argv[8] = (char *)"Everyone:F";
 	argv[9] = NULL;
 
 	rc = libzfs_run_process(argv[0], argv, 0);