ipfilter: Calculate the number of elements in ipf_errors

[cy]

Summary

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

MFC after: 1 day

[emaste]

Or if you wanted you could just use nitems() in find_error instead of IPF_NUM_ERRORS

[cy]

In D53308#1217827, @emaste wrote:

Or if you wanted you could just use nitems() in find_error instead of IPF_NUM_ERRORS

That would require #include <sys/param.h> in interror.c. We don't need the extra definitions (namespace pollution) from param.h.