afs_FreeDiscardedDCache: Avoid assert on error

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.

Change-Id: Id1e77af18461c9804b655c6f91ac90038621a394
Reviewed-on: http://gerrit.openafs.org/8406
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>