devlink: fix "devlink health dump" command without arg

Fix bug when user calls "devlink health dump" without "show" or "clear":
$ devlink health dump
Command "(null)" not found

Put the dump command into a separate helper as it is usual in the rest
of the code. Also, treat no cmd as "show", as it is common for other
devlink objects.

Fixes: 041e6e651a8e ("devlink: Add devlink health dump show command")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
1 file changed