rimson/net: cap outgoing message batch size to prevent seastar packet fragment overflow

rimson/net: cap outgoing message batch size to prevent seastar packet fragment overflow

During heavy backfill, sweep_out_pending_msgs_to_sent() could accumulate
more than 65535 buffer fragments in a single bufferlist, overflowing
seastar's uint16_t packet fragment counter and triggering an assertion.
Limit the sweep to crimson_osd_max_send_buffers fragments
per write; remaining messages are sent in subsequent dispatch iterations.

Fixes: https://tracker.ceph.com/issues/75600
Signed-off-by: Ronen Friedman <rfriedma@redhat.com>