BPF packet pagesize limit

[Ryan Stone]

Do not allocate MJUM9BYTES clusters under any circumstances.  Trying
to allocate them when the system is under memory pressure is
enormously expensive and stands a good chance of livelocking the
system if you try to allocate too many too quickly, even when
allocating with M_NOWAIT.

Honestly, support for clusters > PAGE_SIZE should probably be retired
from the kernel given the significant havoc they cause.