uml: fold mmu_context_skas into mm_context

This patch folds mmu_context_skas into struct mm_context, changing all users
of these structures as needed.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>