rbd: another set of patches

I keep moving simple and simplifying patches to the front of my patch queue. This is my latest set of things that are ready to get reviewed and committed.

-Alex

[PATCH 01/11] rbd: handle locking inside __rbd_client_find()
This simplifies a little code.
[PATCH 02/11] rbd: don't over-allocate space for object prefix
Don't allocate the max required space for the object prefix. This also ensures that the object prefix is either terminated with '\0' or else is maximally-sized.
[PATCH 03/11] rbd: kill incore snap_names_len
Eliminate an unnecessary field in struct rbd_image_header.
[PATCH 04/11] rbd: more cleanup in rbd_header_from_disk()
Rearrange some code, for better logical grouping.
[PATCH 05/11] rbd: move rbd_opts to struct rbd_device
Tie rbd-specific options to the rbd_device, not the client.
[PATCH 06/11] rbd: add read_only rbd map option
[PATCH 07/11] rbd: kill notify_timeout option
These two were done together, in this order, to preserve the argument parsing code. The first adds the ability to map an rbd image read-only. The second eliminates the only other rbd option, which is otherwise unused.
[PATCH 08/11] rbd: bio_chain_clone() cleanups
Simple refactoring.
[PATCH 09/11] rbd: drop needless test in rbd_rq_fn()
Trivial.
[PATCH 10/11] rbd: check for overflow in rbd_get_num_segments()
Make sure we don't exceed 64-bit maximum when adding offset and length.
[PATCH 11/11] rbd: split up rbd_get_segment()
Refactor rbd_get_segment() into three separate functions, which return a segment name, offset, and length.