ftrace: convert unsigned index to signed

Impact: fix to unsigned compared to less than zero

Roel Kluin pointed out that there is a compare of an unsigned number
to less than zero. A previous clean up had the unsigned index set
to -1 for certain cases, but never converted it to signed.

Frederic Weisbecker noticed that another index is used to compare
the above index to and it also needs to be converted to signed.

[
  Converted ftrace_page->index to int from unsigned long as
  Andrew Morton pointed out that there's no need for it to
  stay a long.
]

Reported-by: Roel Kluin <roel.kluin@gmail.com>
Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>