Enables generic KASAN mode.

This mode is supported in both GCC and Clang. With GCC it requires
version 8.3.0 or later. Any supported Clang version is compatible,
but detection of out-of-bounds accesses for global variables is
supported only since Clang 11.

This mode consumes about 1/8th of available memory at kernel start
and introduces an overhead of ~x1.5 for the rest of the allocations.
The performance slowdown is ~x3.

Currently CONFIG_KASAN_GENERIC doesn't work with CONFIG_DEBUG_SLAB
(the resulting kernel does not boot).

source code:

mm/kasan/