tracing: Correctly expand len expressions from __dynamic_array macro

tracing: Correctly expand len expressions from __dynamic_array macro

This fixes expansion of the len argument in __dynamic_array macros.
The previous code from commit 7d536cb3f would not fully evaluate the
expression before multiplying its result by the size of the type.

This went unnoticed because the length stored in the high 16 bits of the
offset (which is the one that was broken here) is only used by
filter_pred_strloc which only acts on strings for which the size of the
type is 1.

Link: http://lkml.kernel.org/r/1393651938-16418-2-git-send-email-filbranden@google.com
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>