cxl/core: Use guard() to replace open-coded down_read/write()

cxl/core: Use guard() to replace open-coded down_read/write()

Some down/up_read() and down/up_write() cases can be replaced by a
guard() simply to drop explicit unlock invoked. It helps to align coding
style with current CXL subsystem's.

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/20250221012453.126366-2-ming.li@zohomail.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>