Make m_get2() never use clusters that are bigger than PAGE_SIZE.

Requested by:	andre, jhb
Sponsored by:	Nginx, Inc.
- Use m_get2() instead of hand allocating.
- No need for u_int cast here.

Sponsored by:	Nginx, Inc.
Use m_get2() + m_align() instead of hand made key_alloc_mbuf(). Code
examination shows, that although key_alloc_mbuf() could return chains,
the callers never use chains, so m_get2() should suffice.

Sponsored by:	Nginx, Inc.
Use m_get() and m_getcl() instead of compat macros.