tcp: use a single counter for limiting the RST rate

Using two counters does not provide any benefit, but it provides an
externally observable signal whether there is a listening port.

Reviewed by: Nick Banks, Peter Lei
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D51440

(cherry picked from commit d31bdbc3661f6535d5ce4bc98c3687e10aa56720)