Linux: work around use of GPL-only symbol kasan_flag_enabled

We may not be able to avoid our code referencing the symbol, but we can
ensure that a symbol of that name is available to the linker during
build, and so not require linking the GPL-exported version.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #18009
Closes #18040