Plug a leak of open files that happens when you exec a suid program

with one of std{in,out,err} open. This helps with the file descriptor
leaks reported on -current. This should probably be merged into 5.2.

Reviewed by:	ru
Tested by:	Bjoern A. Zeeb <bzeeb-lists@lists.zabbadoz.net>

sys/kern/kern_descrip.c

@@ -1730,6 +1730,7 @@ fdcheckstd(td)
 			fp->f_type = DTYPE_VNODE;
 			VOP_UNLOCK(nd.ni_vp, 0, td);
 			devnull = fd;
+			fdrop(fp, td);
 		} else {
 			error = do_dup(td, DUP_FIXED, devnull, i, &retval);
 			if (error != 0)