mm/vmap: Add a notifier for when we run out of vmap address space

vmaps are temporary kernel mappings that may be of long duration.
Reusing a vmap on an object is preferrable for a driver as the cost of
setting up the vmap can otherwise dominate the operation on the object.
However, the vmap address space is rather limited on 32bit systems and
so we add a notification for vmap pressure in order for the driver to
release any cached vmappings.

The interface is styled after the oom-notifier where the callees are
passed a pointer to an unsigned long counter for them to indicate if they
have freed any space.

v2: Guard the blocking notifier call with gfpflags_allow_blocking()
v3: Correct typo in forward declaration and move to head of file

Signed-off-by: Chris Wilson <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Roman Peniaev <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: [email protected]
Cc: [email protected]
Acked-by: Andrew Morton <[email protected]> # for inclusion via DRM
Cc: Joonas Lahtinen <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Joonas Lahtinen <[email protected]>

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