[PATCH 0 of 9] mmu notifier #v12

The difference with #v11 is a different implementation of mm_lock that
guarantees handling signals in O(N). It's also more lowlatency friendly. 

Note that mmu_notifier_unregister may also fail with -EINTR if there are
signal pending or the system runs out of vmalloc space or physical memory,
only exit_mmap guarantees that any kernel module can be unloaded in presence
of an oom condition.

Either #v11 or the first three #v12 1,2,3 patches are suitable for inclusion
in -mm, pick what you prefer looking at the mmu_notifier_register retval and
mm_lock retval difference, I implemented and slighty tested both. GRU and KVM
only needs 1,2,3, XPMEM needs the rest of the patchset too (4, ...) but all
patches from 4 to the end can be deffered to a second merge window.