on in write path

An empty r2tqueue is a valid state. It just means that we have
processed all that there was to do. This patch removes the WARN_ON
that was added when the kfifo changes were merged.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>
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