viced: Avoid endless BCB loop

Without this commit, when we break callbacks for a fid, we loop over
all callbacks for the fid, break a few of them, and then start over.
We do this repeatedly until we run out of callbacks. If a client sees
a callback break, and then establishes a new callback promise while
the fileserver is still breaking callbacks, the fileserver can break
the same callback for the same host again and again. This can continue
forever, if the client establishes its new callback promises quickly
enough.

So to avoid this, when we start breaking callbacks, flag all of the
callback structures that we want to look at. Then when we repeatedly
loop through all of the callbacks for the fid, only look at the
flagged callback structures.

This adds a 'flags' field to struct CallBack, and defines a single
flag, CBFLAG_BREAKING.

This is an alternative fix to the issue also fixed in 843d705c. This
implementation avoids allocating extra memory under locks, and has the
slight benefit of not breaking callbacks that were elsewhere deleted
during the BCB. This comes at the cost of a single extra traversal
through our callback list, and the cost of claiming one of the bits in
the CallBack structure.

Reviewed-on: http://gerrit.openafs.org/10172
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 47124f337b43f8731bfbe3bd71e42d046a4d1075)

Change-Id: I522e0cecd0a9a10bf9eafaae669f4f0005ced893
Reviewed-on: http://gerrit.openafs.org/10755
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>