mmap locking API: initial implementation as rwsem wrappers

This patch series adds a new mmap locking API replacing the existing
mmap_sem lock and unlocks.  Initially the API is just implemente in terms
of inlined rwsem calls, so it doesn't provide any new functionality.

There are two justifications for the new API:

- At first, it provides an easy hooking point to instrument mmap_sem
  locking latencies independently of any other rwsems.

- In the future, it may be a starting point for replacing the rwsem
  implementation with a different one, such as range locks.  This is
  something that is being explored, even though there is no wide concensus
  about this possible direction yet.  (see
  https://patchwork.kernel.org/cover/11401483/)

This patch (of 12):

This change wraps the existing mmap_sem related rwsem calls into a new
mmap locking API.  There are two justifications for the new API:

- At first, it provides an easy hooking point to instrument mmap_sem
  locking latencies independently of any other rwsems.

- In the future, it may be a starting point for replacing the rwsem
  implementation with a different one, such as range locks.

Signed-off-by: Michel Lespinasse <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Daniel Jordan <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Reviewed-by: Laurent Dufour <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Liam Howlett <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Ying Han <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Michel Lespinasse <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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