This adds support for sharing semaphores using kernel syncobjects.

Syncobj backed semaphores are used for any semaphore which is
created with external flags, and when a semaphore is imported,
otherwise we use the current non-kernel semaphores.

Temporary imports from syncobj fd are also available, these
just override the current user until the next wait, when the
temp syncobj is dropped.

v2: allocate more chunks upfront, fix off by one after
previous refactor of syncobj setup, remove unnecessary null
check.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>