ftrace: Allow configuring global trace buffer size (for dump-on-oops)

We have recently turned on ftrace-dump-on-oops for i915's CI and an
issue we have encountered is that the trace buffer size greatly exceeds
the pstore capabilities; we get the tail of the oops but not the
introduction.

Currently the global buffer size is controllable on the cmdline, but at
the request of our CI sysadmin, we would like to add a control to the
Kconfig as well. The rationale being the cmdline carries the temporary
hacks that we want to eradicate, and we want to track the permanent
configuration in .config.

I have kept the Kconfig option hidden from the user as the default
should suffice for the majority of users; reserving the configuration
for those that eschew the cmdline option.

v2: Add an expert prompt to stop the default value overriding .config
changes.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/8029


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>