devlink: Add devlink health dump show command

devlink: Add devlink health dump show command

Add devlink dump show command which displays the last saved dump.
Devlink health saves a single dump. If a dump is not already stored
by the devlink for this reporter, devlink generates a new dump. The dump
can be generated automatically when a reporter reports on an
error or manually by user's request.
The dump's output is defined by the reporter. The command uses the
infra structure for flexible format output introduced in previous patch.
Example:
$ devlink health dump show pci/0000:00:09.0 reporter tx

Signed-off-by: Aya Levin <ayal@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David Ahern <dsahern@gmail.com>