pfsync: fix incorrect enabling of defer mode

When we exposed the PFSYNCF_OK flag to userspace in 5f5bf88949d we
unintentionally caused defer mode to always be enabled.
The ioctl check only looked for nonzero, not for the PFSYNCF_DEFER flag.

Fix this check and ensure ifconfig sets the flag.

Reviewed by: glebius
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33244