Centralize posix_madvise wrapper with errno-aware severity

[KimYannn]

Problem

Five call sites in MMseqs2 log every posix_madvise failure at Debug::ERROR severity, even though posix_madvise is a purely advisory API and every site falls through to continue execution.

On ARM64 kernels with 64KB pages (e.g. NVIDIA Grace, Apple Silicon under some configs), a benign tail-page alignment mismatch on mmap'd databases produces user-visible

posix_madvise returned an error (touchMemory)

lines via stdout/stderr capture in downstream services (e.g. mmseqs gpuserver). This suggests a fault where none exists and is confusing for operators triaging real failures.

The same call sites also discard errno, so when a real I/O problem does occur (EIO, EBADF, ENOMEM) there is no way to tell it apart from the benign cases.

Change

Introduce Util::madviseLogged(addr, len, advice, context) in commons/Util.{h,cpp} which:

• returns 0 when len == 0 or HAVE_POSIX_MADVISE is undefined
• on failure, logs strerror(rc) with the named advice and a caller-supplied context string
• chooses severity by category:
  • SEQUENTIAL hints — always WARNING (pure readahead hint, never functional)
  • WILLNEED + EINVAL — WARNING (alignment / VMA-type edge case, manual touch loop still prefaults)
  • WILLNEED + other errno — ERROR (EIO / EBADF / ENOMEM indicate real problems)

Fallback POSIX_MADV_* macros are provided in Util.h so call sites compile cleanly when HAVE_POSIX_MADVISE is undefined; the helper is a no-op in that build configuration.

All five sites are refactored onto the helper:

• src/commons/Util.cpp (touchMemory)
• src/commons/DBReader.cpp (setSequentialAdvice)
• src/linclust/KmerIndex.h (KmerIndex load)
• src/linclust/kmermatcher.cpp (mergeKmerFilesAndOutput)
• src/prefiltering/Prefiltering.cpp (mergeTargetSplits)

The #ifdef HAVE_POSIX_MADVISE guards previously wrapping each site are now consolidated inside the helper.

Behavior change

• No functional change.

• Severity classification changes as described above.

• Log message format changes: now includes the advice name and strerror() text.

  Before:

  posix_madvise returned an error (touchMemory)
  

  After (WILLNEED + EINVAL on a 64KB-page ARM64 kernel):

  posix_madvise(WILLNEED) failed for touchMemory: Invalid argument
  

Reproduction of original warning

Run any mmseqs workload that loads a database via touchMemory (e.g. mmseqs gpuserver) on an ARM64 host with 64KB pages and a database whose mapped length is not a multiple of the page size. The previous build emits Debug::ERROR; this build emits Debug::WARNING with errno context.