libafs: Remove unecessary parameters to afs_read

libafs: Remove unecessary parameters to afs_read

We were providing additional buffer and length parameters to
afs_read which are now unused, as the necessary information is
contained within the iovec. Just remove these parameters to tidy the
code up a bit.

Change-Id: I2ca80b20e253e0d54be759e6d504961958b40e6e
Reviewed-on: http://gerrit.openafs.org/4464
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>