Pull up following revision(s) (requested by maxv in ticket #833):

sys/kern/uipc_mbuf.c: revision 1.214

Revert my rev1.190, remove the M_READONLY check. The initial code was
correct: what is read-only is the mbuf storage, not the mbuf itself. The
storage contains the packet payload, and never has anything related to
mbufs. So it is fine to remove M_PKTHDR on mbufs that have a read-only
storage.

In fact it was kind of obvious, since several places already manually
remove M_PKTHDR without taking care of the external storage.

(martin)