Make sure to reset transflag back to zero upon succesfully using sendfile()

to transfer a file.

PR: 39362
Submitted by: TANAKA Hiroyuki <kattyo@abk.nu>
MFC after: 1 week

libexec/ftpd/ftpd.c

@@ -1845,6 +1845,7 @@ send_data(FILE *instr, FILE *outstr, off_t blksize, off_t filesize, int isreg)
 				}
 			}
 
+			transflag = 0;
 			reply(226, "Transfer complete.");
 			return (0);
 		}