mptcp: MPTCP_HMAC_TEST should depend on MPTCP

As the MPTCP HMAC test is integrated into the MPTCP code, it can be
built only when MPTCP is enabled.  Hence when MPTCP is disabled, asking
the user if the test code should be enabled is futile.

Wrap the whole block of MPTCP-specific config options inside a check for
MPTCP.  While at it, drop the "default n" for MPTCP_HMAC_TEST, as that
is the default anyway.

Fixes: 65492c5a ("mptcp: move from sha1 (v0) to sha256 (v1)")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>