[RFC] inconsistent semantics of _copy_mc_to_iter()

	In case of failure halfway through the operation we get
very different results depending upon the iov_iter flavour:

	iovec, pipe - advances by the amount actually copied,
	kvec, bvec - does *NOT* advance at all

Which semantics is desired?  AFAICS, the calls can be repeated -
e.g. the loop in dax_iomap_actor() will call dax_copy_to_iter()
again on the short read and with iovec-backed iter it will
try to copy from the place of failure (presumably returning 0
that time around and terminating the loop), while with bvec
or kvec it will go and paste the copies of the same chunk again
until it runs out of destination.
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