block/gluster: return correct error value

block/gluster: return correct error value

Upon error, gluster will call the aio callback function with a
ret value of -1, with errno set to the proper error value.  If
we set the acb->ret value to the return value in the callback,
that results in every error being EPERM (i.e. 1).  Instead, set
it to the proper error result.

Reviewed-by: Niels de Vos <ndevos@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>