tcp: define macros for a couple reclaim thresholds

A following patch is going to implement a similar reclaim schema
for the MPTCP protocol, with different locking.

Let's define a couple of macros for the used thresholds, so
that the latter code will be more easily maintainable.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>