devlink: Add helper function to validate object handler

devlink: Add helper function to validate object handler

Every handler argument validated in two steps, first of which, form
checking, expects identifier is few words separated by slashes.
For device and region handlers just checked if identifier have expected
number of slashes.
Add generic function to do that and make code cleaner & consistent.

Signed-off-by: Dmytro Linkin <dlinkin@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>