[RFG] sfc: nvlog and devlink health

[Edward Cree <ecree.xilinx@gmail.com>]

Solarflare NICs have a flash partition to which the MCPU logs various
 errors, warnings, and other diagnostic info.  We want to expose this
 'nvlog' data, and the best fit we've found so far is devlink health.
Reading it is simple enough — plan is to have a reporter whose diagnose
 method reads the partition and returns the contents (could potentially
 use dump instead but the extra layer of triggering and saving seems
 unnecessary).
The problem is how to clear it (since it fills up after comparatively
 few boots, so when debugging field issues you'll usually need to clear
 it first and then reproduce the issue).
 DEVLINK_CMD_HEALTH_REPORTER_DUMP_CLEAR is no use here, because it only
 clears the kernel-saved copy; it doesn't call any driver method.
The code we've developed internally, that I'm now preparing to submit
 upstream, handles this by having *two* reporters, 'nvlog' and
 'nvlog-clear'; both read the flash in their diagnose method but
 nvlog-clear additionally clears it afterwards.  It works, but it
 doesn't feel very clean.
Is this approach acceptable?  Is there a better way?

-ed