Log errno and thread ID on CreateIOUring failure (facebook#14520)

Summary:

CreateIOUring() silently returns nullptr on failure, discarding the errno
from io_uring_queue_init. This makes it impossible to diagnose why
io_uring initialization fails on specific threads (e.g. ENOMEM from
memlock limits, EINVAL from unsupported flags, EMFILE from fd exhaustion).

Add a fprintf(stderr, ...) that logs strerror, errno, and pthread thread
ID when io_uring_queue_init fails, so failures are diagnosable from logs
without needing to reproduce.

Differential Revision: D98526792

Author: archang19

env/io_posix.h

@@ -8,6 +8,9 @@
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 #pragma once
 #include <errno.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <string.h>
 #if defined(ROCKSDB_IOURING_PRESENT)
 #include <liburing.h>
 #include <sys/uio.h>
@@ -341,6 +344,8 @@ inline struct io_uring* CreateIOUring() {
   flags |= IORING_SETUP_DEFER_TASKRUN;
   int ret = io_uring_queue_init(kIoUringDepth, new_io_uring, flags);
   if (ret) {
+    fprintf(stderr, "CreateIOUring failed: %s (errno=%d), thread=%lu\n",
+            strerror(-ret), -ret, static_cast<unsigned long>(pthread_self()));
     delete new_io_uring;
     new_io_uring = nullptr;
   }