new lwbuf api and mpsafe sf_bufs

[Samuel Greear]

http://gitweb.dragonflybsd.org/~sjg/dragonfly.git/commitdiff/caac5766643042650328f83531dc8fd767df5f60

The basic thought behind the new lwbuf api is that sf_buf's have been
abused more and more as time has passed for purposes that don't
necessarily match the original intent. Most uses of sf_bufs beyond
sendfile are exceedingly short-lived, short enough that they would be
better served by cpu-local caches of kvm space, even if that means a
bit of overlap now and again. lwbuf attempts to match the semantics of
xio, with one addition as suggested by Simon, a sanity check and tlb
invalidation in lwbuf_kva if the lwbuf strays to some other cpu.
Currently lwbuf allocates a page of kvm at a time on objcache object
construction and stores the kvm with the object in objcache when it is
returned to the cache.

sf_bufs, which are still most applicable to sendfile, have been
simplified on top of the lwbuf api, converted to objcache and the
global hash has been protected with a spinlock.

XIO and exec have been modified to use lwbuf's.

Please comment/hate/review/test

Thanks,
Sam