Linux: Include linux/uaccess.h rather than asm/uaccess.h if present

Linux: Include linux/uaccess.h rather than asm/uaccess.h if present

Starting with Linux 4.12 there is a module build error on s390
due to asm/uaccess.h using a macro defined in the common header.
The common header has been around since 2.6.18 and has always
included asm/uaccess.h, so switch to using the common header
whenever it is present.

Change-Id: Iaab0d7652483a2a2b1f144f3e90b6d3b902c146d
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Reviewed-on: https://gerrit.openafs.org/12714
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>