Linux: Use splice to speed up cache storeback

Linux: Use splice to speed up cache storeback

This patch adds a new cache store function for Linux, which uses
splice() and direct access to the page cache, rather than doing
data copies to a temporary buffer between rx and afs. It removes
one copy, and some context switching, from the write codepath.

One side-effect here is that it will delay storebehinds from
returning control to the user. Instead of returning once the first
4k has been successfully transfered, we will wait until a cache
chunk has been transmitted. This is currently unavoidable, as we
can't take the GLOCK within a splice actor.

Change-Id: I5b0284d67febccf099710589908fad18b808332c
Reviewed-on: http://gerrit.openafs.org/903
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>