Always use bigger SECTION_MAP_MASK

It is not necessary to distinguish between different kernel
versions. Kernel commit 2d070eab2e82 merely reuses a previously
unused bit (see clarification in kernel commit def9b71ee651a). The
bit was always zero even before that commit, so it is safe to mask
it off even for kernel versions before 4.13, removing some of the
complexity.

More importantly, makedumpfile fails on kernels which backport the
patch on top of an older version (for example SLES15).

Signed-off-by: Petr Tesarik <ptesarik@suse.com>