Make a distinction between bulk callbacks and brw callbacks - the bulk

Make a distinction between bulk callbacks and brw callbacks - the bulk
callbacks take a ptlrpc_bulk_desc as a parameter, and brw in general has
no idea about that (not that we use the brw callback yet).  Also add a
data parameter for the brw callback, otherwise it is mostly useless.

Some minor prep work for fixing the osc_brw_{read,write} request leak.