net: devlink: Remove overzealous WARN_ON with snapshots

It is possible to trigger this WARN_ON from user space by triggering a
devlink snapshot with an ID which already exists. We don't need both
-EEXISTS being reported and spamming the kernel log.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>