This fixes an LOR in the synfilter implementation.  When a synfilter
is run from _syncache_add(), an INP_RLOCK is obtained while SCH_LOCK
is held from a preceeding syncache lookup.  If another thread calls
syncache_expand() from tcp_input(), syncache_expand() will attempt to
acquire SCH_LOCK while an INP_WLOCK is already held.

To avoid the LOR, when a synfilter is installed, _syncache_add() now
downgrades the INP_WLOCK it acquires on entry to an INP_RLOCK and
holds it until SCH_LOCK is released, which includes across a synfilter
invocation, if any is made.  This also resolves another issue related
to dropping the INP lock, then later reacquiring it and passing the
inp to the synfilter.
Fix munged #ifdef