[PATCH] fix madvise(MADV_DONTNEED) for nonlinear vmas

[PATCH] fix madvise(MADV_DONTNEED) for nonlinear vmas

From: Hugh Dickins <hugh@veritas.com>

Jamie points out that madvise(MADV_DONTNEED) should unmap pages from a
nonlinear area in such a way that the nonlinear offsets are preserved if the
pages do turn out to be needed later after all, instead of reverting them to
linearity: needs to pass down a zap_details block.

(But this still leaves mincore unaware of nonlinear vmas: bigger job.)