afs: avoid afs_GetDownDSlot panic on afs_WriteDCache failure

afs: avoid afs_GetDownDSlot panic on afs_WriteDCache failure

If afs_GetDownDSlot() finds insuffcient free slots in the
afs_freeDSList, it will walk the afs_DLRU attempting to flush and free
eligible dcaches.  However, if an error occurs during the flush to
CacheItems (afs_WriteDCache()), e.g., -EINTR, afs_GetDownDSlot() will
assert.

However, a panic in this case is overkill, since afs_GetDownDSlot() is a
best-effort attempt to free dslots.  The caller (afs_UFSGetDSlot()) will
allocate more dcaches if needed.

Instead:
- Refactor afs_GetDownDSlot() by moving the QRemove() call to after the
afs_WriteDCache logic, so it accompanies the logic that puts the dcache
back on the freelist.  This is safe because we hold the afs_xdcache W
lock for the duration of the routine.
- If afs_WriteDCache() returns an error, return early and let the caller
handle any recovery.

Reviewed-on: https://gerrit.openafs.org/13364
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit d6f52d11c358f71b2c4357cb135e898de7c6277b)

Change-Id: I2630bf04b3e3a88a1fa00f693adf2a77290d47ef
Reviewed-on: https://gerrit.openafs.org/13503
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>