crypto: skcipher - remove redundant clamping to page size

In the case where skcipher_walk_next() allocates a bounce page, that
page by definition has size PAGE_SIZE.  The number of bytes to copy 'n'
is guaranteed to fit in it, since earlier in the function it was clamped
to be at most a page.  Therefore remove the unnecessary logic that tried
to clamp 'n' again to fit in the bounce page.

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