afs: Traverse discard/free dslot list if errors

afs: Traverse discard/free dslot list if errors

Currently, when we pull a dslot off of the discard or free list, we
just try to get the first entry from the list, and panic if we cannot
get it. Instead, traverse through the whole list, trying to find an
entry we can successfully get. This introduces the helper function
afs_GetDSlotFromList to do this traversal.

This does not yet address the case where we cannot get any entry on
the relevant list.

Change-Id: Iedbcffcbeb5dda61fde8e1d526e793f68c3200b3
Reviewed-on: http://gerrit.openafs.org/8376
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>