NFSv4.1: use layout driver in global device cache

pnfs deviceids are unique per server, per layout type.
struct nfs_client is currently used to distinguish deviceids from
different nfs servers, yet these may clash between different layout
types on the same server.  Therefore, use the layout driver associated
with each deviceid at insertion time to look it up, unhash, or
delete it.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>