devlink: let kernel allocate region snapshot id

Currently users have to choose a free snapshot id before
calling DEVLINK_CMD_REGION_NEW. This is potentially racy
and inconvenient.

Make the DEVLINK_ATTR_REGION_SNAPSHOT_ID optional and try
to allocate id automatically. Send a message back to the
caller with the snapshot info.

Example use:
$ devlink region new netdevsim/netdevsim1/dummy
netdevsim/netdevsim1/dummy: snapshot 1

$ id=$(devlink -j region new netdevsim/netdevsim1/dummy | \
       jq '.[][][][]')
$ devlink region dump netdevsim/netdevsim1/dummy snapshot $id
[...]
$ devlink region del netdevsim/netdevsim1/dummy snapshot $id

v4:
 - inline the notification code
v3:
 - send the notification only once snapshot creation completed.
v2:
 - don't wrap the line containing extack;
 - add a few sentences to the docs.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>