s390/kasan: fix insecure W+X mapping warning

Since commit 3b5c3f000c2e ("s390/kasan: move shadow mapping
to decompressor") the decompressor establishes mappings for
the shadow memory and sets initial protection attributes to
RWX. The decompressed kernel resets protection to RW+NX
later on.

In case a shadow memory range is not aligned on page boundary
(e.g. as result of mem= kernel command line parameter use),
the "Checked W+X mappings: FAILED, 1 W+X pages found" warning
hits.

Reported-by: Vasily Gorbik <gor@linux.ibm.com>
Fixes: 557b1970 ("s390/kasan: move shadow mapping to decompressor")
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>