SimpleThrottle: fix -ENOENT checking

The condition was reversed. Rewrite it so it's clear that we're
ignoring -ENOENT only when m_ignore_enoent is set.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Merge pull request #281 from ceph/wip-rbd-rm-enoent

Reviewed-by: Sage Weil <sage@inktank.com>