callout_stop() should return 0 (fail) when the callout is currently
being serviced and indeed unstoppable.

A scenario to reproduce this case is:

• the callout is being serviced and at same time,
• callout_reset() is called on this callout that sets the CALLOUT_PENDING flag and at same time,
• callout_stop() is called on this callout and returns 1 (success) even if the callout is indeed currently running and unstoppable.

This issue was caught up while making r284245 (D2763) workaround, and
was discussed at BSDCan 2015. Once applied the r284245 workaround
is not needed anymore and will be reverted.

Differential Revision: https://reviews.freebsd.org/D3078
Reviewed by: jhb
Sponsored by: Verisign, Inc.