nfs: refactor "need_mount" code out of nfs_try_mount

This looks like pointless refactoring for now, but we'll flesh out
the need_mount case a little more in a later patch.

Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>