sys/mman.h: add MADV_COLLAPSE from linux v6.1

Add madvise flag which performs a best-effort synchronous
collapse of the native pages mapped by the memory range
into Transparent Huge Pages (THPs)
see
  linux commit 7d8faaf155454f8798ec56404faca29a82689c77
  mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse