fuse: Reduce max read size

We are going to introduce parallel processing via coroutines, a maximum read size of 64 MB may be problematic, allowing users of the export to force us to allocate quite large amounts of memory with just a few requests. At least tone it down to 1 MB, which is still probably far more than enough. (Larger requests are split automatically by the FUSE kernel driver anyway.) (Yes, we inadvertently already had parallel request processing due to nested polling before. Better to fix this late than never.) Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Hanna Czenczek <hreitz@redhat.com> Message-ID: <20260309150856.26800-19-hreitz@redhat.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>