Restrict definition of CTL_P1003_1B_MAXID to the kernel

This constant is only used to size an array within the kernel. There are
probably no legitimate uses in userland. Worse, since the kernel's array
could theoretically change size over time, any use of that symbol in
userland wouldn't be forwards compatible to new kernel versions.

Reviewed by: jhb
MFC after: Never
Differential Revision: https://reviews.freebsd.org/D25816