We've found a recurring problem where some userland process would be
stuck spinning at 100% cpu around sbcut_internal(). Inside
sbflush_internal(), sb_ccc reached to about 4GB and before passing it
to sbcut_internal(), we type-cast it from uint to int making it -ve.

The root cause of sockbuf growing this large is unknown. Correct fix
is also not clear but based on mailing list discussions, adding
KASSERTs to panic instead of looping endlessly.

Reviewed by: glebius
Sponsored by: Limelight Networks