Minimise crref() and add a fastpath for cache hits

Minimise crref() and add a fastpath for cache hits

Reduce the number of times we cref() unnecessarily, and add a fastpath
at the beginning of readpage() to handle the cache hit case.

Refactor the Linux implementation of afs_open to provide a directly
callable version that bypasses the OSI layer.

Add a non-blocking variant of ObtainReadLock()

Reviewed-on: http://gerrit.openafs.org/534
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>