NFS: Implement get_nfs_version()

This is a pair for put_nfs_version(), and is used for incrementing the
reference count on the nfs version module. I also updated the callers I
could find who had this hardcoded up until now.

Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>