tcp: rename syncookie_lookup() into syncookie_expand() and make it bool

This function always returns the same pointer it was passed. With new
name and return type the code is easier to understand. Mark the hash
bucket argument as pointer to const, since function doesn't modify it,
just uses value as integer. No functional changes.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D50895