return an error from afs_readdir when out of buffers

return an error from afs_readdir when out of buffers

Instead of silently failing, return EIO from readdir when the
cache manager is unable to allocate a buffer in afs_newslot,
(afs: all buffers locked).

Change-Id: I3d5a5d73ce78db216400cab45a651fd8a49ea0c3
Reviewed-on: http://gerrit.openafs.org/1001
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>