cxl/mbox: Convert poison list mutex to ACQUIRE()

cxl/mbox: Convert poison list mutex to ACQUIRE()

Towards removing all explicit unlock calls in the CXL subsystem, convert
the conditional poison list mutex to use a conditional lock guard.

Rename the lock to have the compiler validate that all existing call sites
are converted.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Alison Schofield <alison.schofield@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250711234932.671292-3-dan.j.williams@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>