cxl/memdev: cxl_memdev_ioctl() cleanup

cxl/memdev: cxl_memdev_ioctl() cleanup

In cxl_memdev_ioctl(), the down_read(&cxl_memdev_rwsem) and
up_read(&cxl_memdev_rwsem) can be replaced by a
guard(rwsem_read)(&cxl_memdev_rwsem), it helps to remove the open-coded
up_read(&cxl_memdev_rwsem). Besides, the local var 'rc' can be also
removed to make the code more cleaner.

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/20250221012453.126366-4-ming.li@zohomail.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>