pf: reduce number of hashing operations when handling source nodes

Reduce number of hashing operations when handling source nodes by always
having a pointer to the hash row mutex in the source node. Provide
macros for handling and asserting the mutex. Calculate the hash only
once in pf_find_src_node() and then use this hash in subsequent
operations.

Cherry-picked from development of D39880

Reviewed by: kp, mjg
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D39888