Fix non-atomic read-modify-write on entry->srtt in adjustsrtt()

Fix non-atomic read-modify-write on entry->srtt in adjustsrtt()

The SRTT update loaded the old value, computed a new one, and stored it
back as separate operations.  Two concurrent callers could each read the
same old value and one update would be silently lost.

Use a CAS loop for the read-modify-write on entry->srtt.  For the aging
path, also CAS on entry->lastage to prevent multiple threads from aging
the same entry in the same second.

(cherry picked from commit 4d15494b943eeb246115d9de891a3e5da3a56df5)