Linux: Simplify keyring compatibility code

Linux: Simplify keyring compatibility code

This introduces a few inline functions in osi_compat.h, to reduce the
number of #ifdefs in the main chunk of code. In particular, we gain
  * afs_linux_key_alloc : to handle all the different key_alloc
                          signatures
  * afs_linux_search_keyring : to handle our two different mechanisms
                               for searching a keyring
  * afs_linux_cred_is_current : will return true if we're using native
				credentials, and the passed creds are also
				those of the current task

Change-Id: I138f3533a7e8e88e04e4b5508158e003882d63ee
Reviewed-on: http://gerrit.openafs.org/1005
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>