MFstable/11 r334730

Ensure net.inet.tcp.syncache.rexmtlimit is limited by TCP_MAXRXTSHIFT.

If the sysctl variable is set to a value larger than TCP_MAXRXTSHIFT+1,
the array tcp_syn_backoff[] is accessed out of bounds.

Discussed with: jtl@
Approved by: re (gjb)
Sponsored by: Netflix, Inc.