devlink: Add health dump {get,clear} commands

Add devlink health dump commands, in order to run an dump operation
over a specific reporter.

The supported operations are dump_get in order to get last saved
dump (if not exist, dump now) and dump_clear to clear last saved
dump.

It is expected from driver's callback for diagnose command to fill it
via the buffer descriptors API. Devlink will parse it and convert it to
netlink nla API in order to pass it to the user.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>