Handling the dynamic charp module parameter requires us to copy it for
the error state, or remember to lock it when reading (in case it used
with 0600).

v2: Use __always_inline and __builtin_strcmp
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170221162619.15954-1-chris@chris-wilson.co.ukReviewed-by: Jani Nikula <jani.nikula@intel.com>