devlink: refactor end checks in devlink_nl_cmd_region_read_dumpit

Clean up after recent fixes, move address calculations
around and change the variable init, so that we can have
just one start_offset == end_offset check.

Make the check a little stricter to preserve the -EINVAL
error if requested start offset is larger than the region
itself.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>