crypto: scatterwalk - add new functions for copying data

Add memcpy_from_sglist() and memcpy_to_sglist() which are more readable
versions of scatterwalk_map_and_copy() with the 'out' argument 0 and 1
respectively.  They follow the same argument order as memcpy_from_page()
and memcpy_to_page() from <linux/highmem.h>.  Note that in the case of
memcpy_from_sglist(), this also happens to be the same argument order
that scatterwalk_map_and_copy() uses.

The new code is also faster, mainly because it builds the scatter_walk
directly without creating a temporary scatterlist.  E.g., a 20%
performance improvement is seen for copying the AES-GCM auth tag.

Make scatterwalk_map_and_copy() be a wrapper around memcpy_from_sglist()
and memcpy_to_sglist().  Callers of scatterwalk_map_and_copy() should be
updated to call memcpy_from_sglist() or memcpy_to_sglist() directly, but
there are a lot of them so they aren't all being updated right away.

Also add functions memcpy_from_scatterwalk() and memcpy_to_scatterwalk()
which are similar but operate on a scatter_walk instead of a
scatterlist.  These will replace scatterwalk_copychunks() with the 'out'
argument 0 and 1 respectively.  Their behavior differs slightly from
scatterwalk_copychunks() in that they automatically take care of
flushing the dcache when needed, making them easier to use.

scatterwalk_copychunks() itself is left unchanged for now.  It will be
removed after its callers are updated to use other functions instead.

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

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