tracing: Keep the predicate limit separate from program capacity

In the Linux kernel, the following vulnerability has been resolved:

tracing: Keep the predicate limit separate from program capacity

predicate_parse() adds two to nr_preds to reserve the terminal TRUE
and FALSE entries. It also uses that increased value as the limit for
parsed predicates. If the initial count is smaller than the number
actually parsed, the program can fill the allocation and subsequent
accesses can go past its end.

Keep nr_preds as the predicate limit and use a separate value for the
allocation size. This leaves space for both terminal entries and a
zeroed entry when parsing stops at the limit.