NFS: Move details of nfs4_get_sb() to a helper

Clean up: Refactor nfs4_get_sb() to allow its guts to be invoked by
nfs_get_sb().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>