crypto: scatterwalk - add new functions for skipping data

crypto: scatterwalk - add new functions for skipping data

Add scatterwalk_skip() to skip the given number of bytes in a
scatter_walk.  Previously support for skipping was provided through
scatterwalk_copychunks(..., 2) followed by scatterwalk_done(), which was
confusing and less efficient.

Also add scatterwalk_start_at_pos() which starts a scatter_walk at the
given position, equivalent to scatterwalk_start() + scatterwalk_skip().
This addresses another common need in a more streamlined way.

Later patches will convert various users to use these functions.

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