devlink: Add helper function for safely copy string param

Devlink string param buffer is allocated at the size of
DEVLINK_PARAM_MAX_STRING_VALUE. Add helper function which makes sure
this size is not exceeded.
Renamed DEVLINK_PARAM_MAX_STRING_VALUE to
__DEVLINK_PARAM_MAX_STRING_VALUE to emphasize that it should be used by
devlink only. The driver should use the helper function instead to
verify it doesn't exceed the allowed length.

Signed-off-by: Moshe Shemesh <moshe@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>