Fix an unnecessary/incorrect check in the PKTOPT_EXTHDRCPY macro.

This macro allocates memory and, if malloc does not return NULL, copies
data into the new memory. However, it doesn't just check whether malloc
returns NULL. It also checks whether we called malloc with M_NOWAIT. That
is not necessary.

While it may be that malloc() will only return NULL when the M_NOWAIT flag
is set, we don't need to check for this when checking malloc's return
value. Further, in this case, the check was not completely accurate,
because it checked for flags == M_NOWAIT, rather than treating it as a bit
field and checking for (flags & M_NOWAIT).

Reviewed by: ae
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D10942