afs: clean afs_osi_Alloc() usage

afs: clean afs_osi_Alloc() usage

Add asserts for any failures cases not explicitly handled and remove
any casting.

Reviewed-on: http://gerrit.openafs.org/3012
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 0e8cce457763b131de48395a9beed889fd529c1f)

Change-Id: I23497ea7a75a66c01ba7b1013e01dfaa005c9892
Reviewed-on: http://gerrit.openafs.org/7936
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>