Use __always_inline in compilation units that have instrumentation
disabled (KASAN_SANITIZE_foo.o := n) for KASAN, like it is done for
KCSAN.

Also, add common documentation for KASAN and KCSAN explaining the
attribute.

 [ bp: Massage commit message. ]
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20200521142047.169334-12-elver@google.com