DIOCSETREASS stored V_pf_status.reass twice with no lock, first the
flags it was given and then, if reassembly was not among them, zero.
pf_normalize_ip() and pf_normalize_ip6() read the flags with the rules
lock held for reading, so a packet could see the intermediate value.
Compute the flags first and store them once, with the rules lock held
for writing, as DIOCSETDEBUG and DIOCSETHOSTID do.

Fixes: 39282ef356db ("pf: backport OpenBSD syntax of "scrub" option for "match" and "pass" rules")
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")