commit dad41997 upstream.

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.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
[ kamal: backport to 4.2-stable: context ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>