tcp: improve credential handling in syncache

When adding a syncache entry, take a reference count of the
credentials while the inp is still locked.
Thanks to markj@ for providing a hint regarding the root cause.

Reported by: David Marker
Reviewed by: glebius
Tested by: David Marker
Fixes: cbc9438f0505 ("tcp: improve ref count handling when processing SYN")
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D53380