nfs: add minor version to nfs_server_key for fscache

An NFS client that mounts multiple exports from the same NFS
server with higher NFSv4 versions disabled (i.e. 4.2) and without
forcing a specific NFS version results in fscache index cookie
collisions and the following messages:
[  570.004348] FS-Cache: Duplicate cookie detected

Each nfs_client structure should have its own fscache index cookie,
so add the minorversion to nfs_server_key.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=200145


Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>