rbd: cleanup series

This series includes a bunch of relatively small cleanups. They're grouped a bit below, but they apply together in this sequence and the later ones may have dependencies on those earlier in the series.

Summaries:
[PATCH 01/12] rbd: drop extra header_rwsem init [PATCH 02/12] rbd: simplify __rbd_remove_all_snaps() [PATCH 03/12] rbd: clean up a few dout() calls [PATCH 04/12] ceph: define snap counts as u32 everywhere
These four are very simple and straightforward cleanups.

[PATCH 05/12] rbd: snapc is unused in rbd_req_sync_read() [PATCH 06/12] rbd: drop rbd_header_from_disk() gfp_flags parameter [PATCH 07/12] rbd: drop rbd_dev parameter in snap functions [PATCH 08/12] rbd: drop rbd_req_sync_exec() "ver" parameter
These four each drop an unused argument from a function.

[PATCH 09/12] rbd: have __rbd_add_snap_dev() return a pointer [PATCH 10/12] rbd: make rbd_create_rw_ops() return a pointer
These two each change a function so it returns a pointer rather than filling in the address of a provided pointer.

[PATCH 11/12] rbd: always pass ops array to rbd_req_sync_op() [PATCH 12/12] rbd: fixes in rbd_header_from_disk()
These comprise slightly more involved refactoring of the code, but as with the rest of the patches in this series, there should be no functional difference as a result.

-Alex