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>
(cherry picked from commit 584f865ae116185087176715ced014b625f04e82)

Conflicts:
	src/librbd/api/Io.cc [ commit 37057ba2f6bb ("librbd: pass
	  image_extents to create_{discard,write_same}()") and ImageArea
	  support not in pacific ]
	src/librbd/cache/ImageWriteback.cc [ ditto ]
	src/librbd/io/ImageDispatchSpec.h [ ditto ]
	src/librbd/crypto/luks/FormatRequest.cc [ ImageArea support not
	  in pacific ]
	src/test/librbd/io/test_mock_ImageRequest.cc [ ditto ]
	src/test/librbd/journal/test_mock_Replay.cc [ ditto ]
	src/librbd/crypto/luks/FlattenRequest.cc [ commit 84601d68a74f
	  ("librbd/crypto: add format-specific flattening operation")
	  not in pacific ]