• Substitute sbdrop_internal() with sbcut_internal(). The latter doesn't free mbufs, but return chain of free mbufs to a caller. Caller can either reuse them or return to allocator in a batch manner.
• Implement sbdrop()/sbdrop_locked() as a wrapper around sbcut_internal().
• Expose sbcut_locked() for outside usage.

Sponsored by: Netflix
Sponsored by: Nginx, Inc.
Approved by: re (marius)