vhost/vsock: support MSG_EOR bit processing

vhost/vsock: support MSG_EOR bit processing

'MSG_EOR' handling has similar logic as 'MSG_EOM' - if bit present
in packet's header, reset it to 0. Then restore it back if packet
processing wasn't completed. Instead of bool variable for each
flag, bit mask variable was added: it has logical OR of 'MSG_EOR'
and 'MSG_EOM' if needed, to restore flags, this variable is ORed
with flags field of packet.

Signed-off-by: Arseny Krasnov <arseny.krasnov@kaspersky.com>
Link: https://lore.kernel.org/r/20210903123238.3273526-1-arseny.krasnov@kaspersky.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>