libshare: nfs: set export file 644

The shares are publicly known anyway and can be interrogated by any
user, so this is a debugging aid more than anything.

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/nfs.c

@@ -144,6 +144,7 @@ nfs_fini_tmpfile(const char *exports, struct tmpfile *tmpf)
 		return (SA_SYSTEM_ERR);
 	}
 
+	(void) fchmod(fileno(tmpf->fp), 0644);
 	fclose(tmpf->fp);
 	return (SA_OK);
 }