ipfilter: Calculate the number of elements in ipf_errors

It serves no purpose to manually manage the IPF_NUM_ERRORS count.
Calculate it instead.

Reviewed by:		emaste, markj
MFC after:		1 week
Differential revision:	https://reviews.freebsd.org/D53308

sbin/ipf/libipf/interror.c

@@ -17,7 +17,7 @@ typedef	struct	{
 
 static ipf_error_entry_t *find_error(int);
 
-#define	IPF_NUM_ERRORS	477
+#define	IPF_NUM_ERRORS	sizeof(ipf_errors) / sizeof(ipf_error_entry_t)
 
 /*
  * NO REUSE OF NUMBERS!
@@ -25,7 +25,7 @@ static ipf_error_entry_t *find_error(int);
  * IF YOU WANT TO ADD AN ERROR TO THIS TABLE, _ADD_ A NEW NUMBER.
  * DO _NOT_ USE AN EMPTY NUMBER OR FILL IN A GAP.
  */
-static ipf_error_entry_t ipf_errors[IPF_NUM_ERRORS] = {
+static ipf_error_entry_t ipf_errors[] = {
 	{	1,	"auth table locked/full" },
 	{	2,	"" },
 	{	3,	"copyinptr received bad address" },