NFSv4.1 provide mount option to toggle trunking discovery

Introduce a new mount option -- trunkdiscovery,notrunkdiscovery -- to
toggle whether or not the client will engage in actively discovery
of trunking locations.

v2 make notrunkdiscovery default

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Fixes: 1976b2b3

 ("NFSv4.1 query for fs_location attr on a new file system")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>