inpcb: Remove NULL checks of credential references

inpcb: Remove NULL checks of credential references

Some auditing of the code shows that "cred" is never non-NULL in these
functions, either because all callers pass a non-NULL reference or
because they unconditionally dereference "cred".  So, let's simplify the
code a bit and remove NULL checks.  No functional change intended.

Reviewed by:	glebius
MFC after:	1 week
Sponsored by:	Modirum MDPay
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D37025