librbd: stop passing IOContext to image dispatch write methods

librbd: stop passing IOContext to image dispatch write methods

This is a major footgun since any value passed e.g. at the API layer
may be stale by the time we get to object dispatch.  All callers are
passing the IOContext returned by get_data_io_context() for their
ImageCtx anyway, highlighting that the parameter is fictitious.

Only the read method can meaningfully take IOContext.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>