MAINTAINERS: Add entry for MMU GATHER AND TLB INVALIDATION

MAINTAINERS: Add entry for MMU GATHER AND TLB INVALIDATION

We recently had to debug a TLB invalidation problem on the munmap()
path, which was made more difficult than necessary because:

  (a) The MMU gather code had changed without people realising
  (b) Many people subtly misunderstood the operation of the MMU gather
      code and its interactions with RCU and arch-specific TLB invalidation
  (c) Untangling the intended behaviour involved educated guesswork and
      plenty of discussion

Hopefully, we can avoid getting into this mess again by designating a
cross-arch group of people to look after this code. It is not intended
that they will have a separate tree, but they at least provide a point
of contact for anybody working in this area and can co-ordinate any
proposed future changes to the internal API.

Cc: Peter Zijlstra <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Will Deacon <[email protected]>