ref_tracker: have callers pass output function to pr_ostream()

In a later patch, we'll be adding a 3rd mechanism for outputting
ref_tracker info via seq_file. Instead of a conditional, have the caller
set a pointer to an output function in struct ostream. As part of this,
the log prefix must be explicitly passed in, as it's too late for the
pr_fmt macro.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20250618-reftrack-dbgfs-v15-3-24fc37ead144@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>