Use readpage, not read for fastpath access

Use readpage, not read for fastpath access

Modify the fast path case so that it uses readpage(), rather than read()
to access data in the cache. This removes a lot of the hidden, uncessary
work that the kernel was doing behind the scenes, and takes advantage of
the fact that we know a page read will always result in a page read
against the backing cache.

Reviewed-on: http://gerrit.openafs.org/535
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>