libafs: Remove afs_read duplication

libafs: Remove afs_read duplication

The disk cache and memcache afs_read functions are effectively
duplicates of each other. Abstract out the common code into a generic
afs_read() function, and put the cache type specific code into
UFSReadUIO (there is already a MemReadUIO which contains the code
necessary for the memcache).

Change-Id: Ic66242fa4695a146ac874a82bd48a7c1f1f412a6
Reviewed-on: http://gerrit.openafs.org/4463
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>