tcp: remove incorrect reset of SACK variable in PRR

tcp: remove incorrect reset of SACK variable in PRR
tcp: Add prr_out in preparation for PRR/nonSACK and LRD

Reviewed by:   #transport, rrs, tuexen, kbowling
Approved by:   #re (gjb)
PR:            253848
MFC after:     3 days
Sponsored by:  NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29083
Differential Revision: https://reviews.freebsd.org/D29058

(cherry picked from commit 4a8f3aad37dd35c905e34b64b022d60844ba8d01)
(cherry picked from commit d90bba73a2e43fa12ea19425d101df4c488c1070)

(cherry picked from commit e53138694aa41c24c17847afe959225ce0eeff91)
(cherry picked from commit 71cc98ba735283b8720c9031eb87810f3d3d96a0)