Use m_getcl() instead of hand made allocation.

Sponsored by:	Nginx, Inc.
- Use m_getcl() instead of hand allocating.
- Use m_get()/m_gethdr() instead of macros.
- Remove superfluous cleaning of mbuf fields after allocation.

Sponsored by:	Nginx, Inc.
- Use m_getcl() instead of hand allocating.
- Convert panic() to KASSERT.
- Remove superfluous cleaning of mbuf fields after allocation.
- Add comment on possible use of m_get2() here.

Sponsored by:	Nginx, Inc.
- Use m_getcl() instead of hand allocating.

Sponsored by:	Nginx, Inc.
Use m_get/m_gethdr instead of compat macros.

Sponsored by:	Nginx, Inc.
Add my copyright for the 2012 year work, in particular vn_io_fault()
and f_offset locking.  Add required Foundation notice for r248319.

Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
We can, and should use M_WAITOK here.

Sponsored by:	Nginx, Inc.