cxl/core: cxl_mem_sanitize() cleanup

cxl/core: cxl_mem_sanitize() cleanup

In cxl_mem_sanitize(), the down_read() and up_read() for
cxl_region_rwsem can be simply replaced by a guard(rwsem_read), and the
local variable 'rc' can be removed.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Li Ming <ming.li@zohomail.com>
Link: https://patch.msgid.link/20250221012453.126366-3-ming.li@zohomail.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>