The m_extadd() can fail due to memory allocation failure, thus:
- Make it return int, not void.
- Add wait parameter.
- Update MEXTADD() macro appropriately, defaults to M_NOWAIT, as
  before this change.

Sponsored by:	Nginx, Inc.
In kern_sendfile() use m_extadd() instead of MEXTADD() macro, supplying
appropriate wait argument and checking return value. Before this change
m_extadd() could fail, and kern_sendfile() ignored that.

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

Sponsored by:	Nginx, Inc.
Use m_get2() to get mbuf of appropriate size.

Sponsored by:	Nginx, Inc.
Simplify vm_page_is_valid().

Sponsored by:	EMC / Isilon storage division
Reviewed by:	alc
- Use m_get2() instead of nfsm_reqhead().
- Use m_get(), m_getcl() instead of historic macros.

Sponsored by:	Nginx, Inc.