Merge branch 'allocate-multiple-skbuffs-on-tx'

Arseniy Krasnov says:

====================
allocate multiple skbuffs on tx

This adds small optimization for tx path: instead of allocating single
skbuff on every call to transport, allocate multiple skbuff's until
credit space allows, thus trying to send as much as possible data without
return to af_vsock.c.

Also this patchset includes second patch which adds check and return from
'virtio_transport_get_credit()' and 'virtio_transport_put_credit()' when
these functions are called with 0 argument. This is needed, because zero
argument makes both functions to behave as no-effect, but both of them
always tries to acquire spinlock. Moreover, first patch always calls
function 'virtio_transport_put_credit()' with zero argument in case of
successful packet transmission.
====================

Link: https://lore.kernel.org/r/b0d15942-65ba-3a32-ba8d-fed64332d8f6@sberdevices.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>