Fix recvmmsg(2) implementation

There were two problems discovered with the `recvmmsg(2)` implementation
that this changeset attempts to fix:

1. As mentioned in nix-rust/issues/1325, `recvmmsg(2)` can return fewer
   messages than requested, and
2. Passing the return value of `recvmmsg(2)` as the number of bytes in
   the messages received is incorrect.

This changeset incorporates the proposed fix from nix-rust/issues/1325,
as well as passing the correct value (`mmsghdr.msg_len`) for the number
of bytes in a given message.

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