tracing: Allow multiple hitcount values in histograms

The hitcount is treated specially in the histograms - since it's
always expected to be there regardless of whether the user specified
anything or not, it's always added as the first histogram value.

Currently the code doesn't allow it to be added more than once as a
value, which is inconsistent with all the other possible values.  It
would seem to be a pointless thing to want to do, but other features
being added such as percent and graph modifiers don't work properly
with the current hitcount restrictions.

Fix this by allowing multiple hitcounts to be added.

Link: https://lore.kernel.org/linux-trace-kernel/166610812248.56030.16754785928712505251.stgit@devnote2



Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Tested-by: Tom Zanussi <zanussi@kernel.org>