cxl/region: Consolidate cxl_decoder_kill_region() and cxl_region_detach()

Both detach_target() and cxld_unregister() want to tear down a cxl_region
when an endpoint decoder is either detached or destroyed.

When a region is to be destroyed cxl_region_detach() releases
cxl_region_rwsem unbinds the cxl_region driver and re-acquires the rwsem.

This "reverse" locking pattern is difficult to reason about, not amenable
to scope-based cleanup, and the minor differences in the calling context of
detach_target() and cxld_unregister() currently results in the
cxl_decoder_kill_region() wrapper.

Introduce cxl_decoder_detach() to wrap a core __cxl_decoder_detach() that
serves both cases. I.e. either detaching a known position in a region
(interruptible), or detaching an endpoint decoder if it is found to be a
member of a region (uninterruptible).

Cc: Davidlohr Bueso <[email protected]>
Cc: Jonathan Cameron <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: Alison Schofield <[email protected]>
Cc: Vishal Verma <[email protected]>
Cc: Ira Weiny <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Dan Williams <[email protected]>
Reviewed-by: Fabio M. De Francesco <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
Reviewed-by: Jonathan Cameron <[email protected]>
Reviewed-by: Alison Schofield <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Dave Jiang <[email protected]>

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