NFS: Rename get_nfs_version() -> find_nfs_version()

We have a put_nfs_version() that handles refcounting on the nfs version
module, but get_nfs_version() does much more work to find a version
module based on version number. Let's change 'get' to 'find' to better
match what it's doing.

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