crypto: scompress - use sgl_alloc() and sgl_free()

Use the sgl_alloc() and sgl_free() functions instead of open coding
these functions.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jens Axboe <axboe@kernel.dk>