MADV_SOFT_OFFLINE

[enh]

i'll be honest, i've never really understood glibc's relationship to
the linux kernel headers ... but it seems unlikely to be deliberate
that MADV_SOFT_OFFLINE is only available via `#include
<asm-generic/mman-common.h>` for glibc users?

i think all the other MADV_ constants are available, including its
close relative [in the sense of "guarded by the same kernel config"]
MADV_HWPOISON.