pstore: Use global ftrace filters for function trace filtering

pstore: Use global ftrace filters for function trace filtering

Currently, pstore doesn't have any filters setup for function tracing.
This has the associated overhead and may not be useful for users looking
for tracing specific set of functions.

ftrace's regular function trace filtering is done writing to
tracing/set_ftrace_filter however this is not available if not requested.
In order to be able to use this feature, the support to request global
filtering introduced earlier in the series should be requested before
registering the ftrace ops. Here we do the same.

Signed-off-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>