cxl/region: Convert cxl_pmem_region_alloc to scope-based resource management

cxl/region: Convert cxl_pmem_region_alloc to scope-based resource management

A recent bugfix to cxl_pmem_region_alloc() to fix an
error-unwind-memleak [1], highlighted a use case for scope-based resource
management.

Delete the goto for releasing @cxl_region_rwsem, and return error codes
directly from error condition paths.

The caller, devm_cxl_add_pmem_region(), is no longer given @cxlr_pmem
directly it must retrieve it from @cxlr->cxlr_pmem. This retrieval from
@cxlr was already in place for @cxlr->cxl_nvb, and converting
cxl_pmem_region_alloc() to return an int makes it less awkward to handle
no_free_ptr().

Cc: Li Zhijian <lizhijian@fujitsu.com>
Reported-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Closes: http://lore.kernel.org/r/20240430174540.000039ce@Huawei.com
Link: http://lore.kernel.org/r/20240428030748.318985-1-lizhijian@fujitsu.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/171451430965.1147997.15782562063090960666.stgit@dwillia2-xfh.jf.intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>