pnfs/objlayout: fix endianess annotation in objio_alloc_deviceid_node

The kbuild test robot complained about a new sparse warning in
objio_alloc_deviceid_node, but it turns out that this was just a moved
reference to an existing variable.  Fix it to have the right big endian
annotated type.

Note that there are some other endianess issues in this file that I didn't
bother to sort out as they involve global headers.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>