Linux: Don't hide memory management

Linux: Don't hide memory management

Given that we now only switch from user space into kernel space in
one place, don't hide the mechanism we use to do so behind macros.
This makes it much easier to visually confirm the correctness of the
code.

Change-Id: Ie52e071c81a9178c792be1eaa7e36d8453ebb319
Reviewed-on: http://gerrit.openafs.org/1820
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Dan Hyde <drh@umich.edu>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>