krbd: match new with delete, not free()

struct krbd_ctx is allocated with new, use delete to get rid of it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
krbd: fix sysfs path in the comment

It's "/sys/bus/rbd/devices/<id>", but libudev works with devices and
not busses, so it's really "/sys/devices/rbd/<id>/".

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1790 from ceph/wip-krbd-fixes

Two minor krbd fixes

Reviewed-by: Sage Weil <sage@inktank.com>