afs_FreeDiscardedDCache: Avoid assert on error

Currently afs_FreeDiscardedDCache will assert if it cannot read in any
discarded dcache entry to free. Return an error instead of asserting,
so the caller can figure out what to do about the error.

Adjust the callers to handle the error, or panic.
afs_MaybeFreeDiscardedDCache still just panics anyway, as making it
handle the error gracefully is beyond the scope of this commit, and is
work for another day.

This changes afs_FreeDiscardedDCache to return an int.

Reviewed-on: http://gerrit.openafs.org/8406
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 866a9fbbdd89735ab76a70cb6d6523db2b98e4c9)

Change-Id: I42a3be4938e5a43c394475653075f9e03a9e7975
Reviewed-on: http://gerrit.openafs.org/10357
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>