crypto: scatterwalk - add new functions for iterating through data

Add scatterwalk_next() which consolidates scatterwalk_clamp() and
scatterwalk_map().  Also add scatterwalk_done_src() and
scatterwalk_done_dst() which consolidate scatterwalk_unmap(),
scatterwalk_advance(), and scatterwalk_done() or scatterwalk_pagedone().
A later patch will remove scatterwalk_done() and scatterwalk_pagedone().

The new code eliminates the error-prone 'more' parameter.  Advancing to
the next sg entry now only happens just-in-time in scatterwalk_next().

The new code also pairs the dcache flush more closely with the actual
write, similar to memcpy_to_page().  Previously it was paired with
advancing to the next page.  This is currently causing bugs where the
dcache flush is incorrectly being skipped, usually due to
scatterwalk_copychunks() being called without a following
scatterwalk_done().  The dcache flush may have been placed where it was
in order to not call flush_dcache_page() redundantly when visiting a
page more than once.  However, that case is rare in practice, and most
architectures either do not implement flush_dcache_page() anyway or
implement it lazily where it just clears a page flag.

Another limitation of the old code was that by the time the flush
happened, there was no way to tell if more than one page needed to be
flushed.  That has been sufficient because the code goes page by page,
but I would like to optimize that on !HIGHMEM platforms.  The new code
makes this possible, and a later patch will implement this optimization.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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