quorum: Inline quorum_fifo_aio_cb()

Inlining the function removes some boilerplace code and replaces
recursion by a simple loop, so the code becomes somewhat easier to
understand.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
