env/posix: improve io_uring init failure diagnostics in ReadAsync

[krhancoc]

Problem

When io_uring_queue_init fails with ENOMEM (memlock limit exhausted — common on systems running many concurrent threads), CreateIOUring returns
nullptr and ReadAsync propagates a generic IOStatus::NotSupported("ReadAsync") with no indication of why the ring creation failed. This makes the failure opaque and hard to diagnose.

Root Cause Chain

 MultiGet(async_io=true)
   → MultiGetFromSSTCoroutine (per SST file)
     → FilePrefetchBuffer::ReadAsync
       → PosixRandomAccessFile::ReadAsync
         → CreateIOUring() → io_uring_queue_init(256, ring, flags)
           ← ENOMEM (memlock exhausted)
         ← returns null
       ← returns IOStatus::NotSupported("ReadAsync")   ← opaque, no errno
     ← per-key error propagated to caller

Fix

• CreateIOUring now accepts an optional int* err_out parameter. On failure it captures the errno from io_uring_queue_init, logs a diagnostic to stderr
  (with a memlock-specific hint when errno is ENOMEM), and — if the caller passes a pointer — fills it with the raw errno so the caller can respond
  appropriately.
• PosixRandomAccessFile::ReadAsync now uses err_out to return a typed IOStatus:
• ENOMEM → IOStatus::IOError(...) with a message explaining the memlock limit
• ENOSYS / EPERM → IOStatus::NotSupported(...) (kernel doesn't support io_uring)
• Other errno → IOStatus::IOError(...) with the raw errno string
• PosixRandomAccessFile::MultiRead is unchanged in behavior — it already falls back to synchronous reads when ring init fails. It calls CreateIOUring()
  with no argument (default nullptr), relying on the internal stderr logging.

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