Impact: improve out-of-range fixmap index debugging

Commit "1b42f516"
defined the __this_fixmap_does_not_exist() function
with a WARN_ON(1) in it.

This causes the linker to not report an error when
__this_fixmap_does_not_exist() is called with a
non-constant parameter.

Ingo defined __this_fixmap_does_not_exist() because he
wanted to get virt addresses of fix memory of nest level
by non-constant index.

But we can fix this and still keep the link-time check:

We can get the four slot virt addresses on link time and
store them to array slot_virt[].

Then we can then refer the slot_virt with non-constant index,
in the ioremap-leak detection code.
Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
LKML-Reference: <49B2075B.4070509@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>