To avoid a kernel panic provoked by a NULL pointer dereference,

do not clear the `sb_sel' member of the sockbuf structure
while invalidating the receive sockbuf in sorflush(), called
from soshutdown().

The panic was reproduceable from user land by attaching a knote
with EVFILT_READ filters to a socket, disabling further reads
from it using shutdown(2), and then closing it.  knote_remove()
was called to remove all knotes from the socket file descriptor
by detaching each using its associated filterops' detach call-
back function, sordetach() in this case, which tried to remove
itself from the invalidated sockbuf's klist (sb_sel.si_note).

PR:	kern/54331

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