devlink: Hang reporter's dump method on a dumpit cb

The devlink health reporter provides a dump method on an error. Dump
may contain a large amount of data, in this case doit cb isn't sufficient.
This is because the user side is blocking and doesn't allow draining of
the socket until the socket runs out of buffers. Using dumpit cb
is the correct way to go.
Please note that thankfully the dump op is not yet implemented in any
driver and therefore this change is not breaking userspace.

Fixes: 35455e23

 ("devlink: Add health dump {get,clear} commands")
Signed-off-by: Aya Levin <ayal@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>