iomap: split iomap_iter() logic into iomap_iter_next()

In preparation for changing iomap to use an in-iter (->iomap_next())
model, move the iomap_iter() logic out into the new iomap_iter_next()
helper function.

iomap_iter_next() is added as an inlined helper so it can be called
directly by ->iomap_next() implementations where the begin()/end()
callbacks can be direct calls.

The DEFINE_IOMAP_ITER_NEXT() and DEFINE_IOMAP_ITER_NEXT_END() macros are
also provided to generate the boilerplate ->iomap_next() wrapper
functions that simply forward to iomap_iter_next() with the appropriate
begin/end callbacks. DEFINE_IOMAP_ITER_NEXT() is for the common case
where there is no end() callback. DEFINE_IOMAP_ITER_NEXT_END() is for
the case where there is an explicit end() callback.

No functional change intended. The one would-be behavioral difference is
that on the iomap_end() error path (ret < 0 && !advanced), the old code
returned with iter.status left as the caller's last value whereas the
new code zeroes it, but this is not observable in practice as there are
no in-tree callers that read iter.status after the iteration loop.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Fengnan Chang <changfengnan@bytedance.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://patch.msgid.link/20260729192737.3190206-3-joannelkoong@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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