Fix unused variable warning in sctp_timer.c
With clang 15, the following -Werror warning is produced:
sys/netinet/sctp_timer.c:510:6: error: variable 'recovery_cnt' set but not used [-Werror,-Wunused-but-set-variable] int recovery_cnt = 0; ^
The 'recovery_cnt' variable is only used when INVARIANTS is undefined.
Ensure it is only declared and set in that case.
MFC after: 3 days
(cherry picked from commit 5bfd8cf3691381c19296d76d7944d6c1df5f4a75)