rpc.mountd: Change nfs_client->m_hostname to be a dynamically-allocated string

Change nfs_client->m_hostname to be dynamically allocated rather than a
fixed length array of size NFSCLNT_IDMAX. This also adds a bit of
micro-optimization in a few places since it reduces the amount of string
copying that needs to be done.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.de>