crypto: algif_skcipher - Do not set MAY_BACKLOG on the async path

The async path cannot use MAY_BACKLOG because it is not meant to
block, which is what MAY_BACKLOG does.  On the other hand, both
the sync and async paths can make use of MAY_SLEEP.

Cc: stable@vger.kernel.org
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>