filedesc: microoptimize fget_unlocked by getting rid of fd < 0 branch

Casting fd to an unsigned type simplifies fd range coparison to mere checking
if the result is bigger than the table.

sys/kern/kern_descrip.c

@@ -2342,7 +2342,7 @@ fget_unlocked(struct filedesc *fdp, int fd, cap_rights_t *needrightsp,
 #endif
 
 	fdt = fdp->fd_files;
-	if (fd < 0 || fd >= fdt->fdt_nfiles)
+	if ((u_int)fd >= fdt->fdt_nfiles)
 		return (EBADF);
 	/*
 	 * Fetch the descriptor locklessly.  We avoid fdrop() races by