Saying that we are called with an (un)finished or corrupted sbuf
is like saying "error" (though [un]finished is an extra hint).
Add the pointer to the sbuf so one could easily check it,
add the flags we are checking so one could see quickly about
a possible state mismatch.
Given we already dereference the pointer in the KASSERT there is no
extra harm adding it to the message.
Found while: testing lindebugfs changes
Sponsored by: The FreeBSD Foundation
MFC after: 3 days