kmemtrace: allow kmemtrace to be enabled after boot

The kmemtrace_init() function returns early if kmemtrace is disabled at boot
causing kmemtrace_setup_late() to also bail out on NULL channel. This has the
unfortunate side effect that none of the debugfs files needed to enable
kmemtrace after boot are created.

Cc: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>