switch_root: improve statfs->f_type portability

__SWORD_TYPE is not available everywhere, for example it's not defined
by musl libc. It also seems that __SWORD_TYPE is not used for f_type
on some architectures (s390x).

Reported-by: Natanael Copa <ncopa@alpinelinux.org>
Signed-off-by: Karel Zak <kzak@redhat.com>