pflog: align header to 4 bytes, not 8

6d4baa0d01 incorrectly rounded the lenght of the pflog header up to 8
bytes, rather than 4.

PR: 261566
Reported by: Guy Harris <gharris@sonic.net>
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 4daa31c10867b133bdc2a424e1e60d280384dc56)