linux: don't use io_uring on pre-5.10.186 kernels (#4093)

Those kernels have a known resource consumption bug where the sqpoll
thread busy-loops.

Fixes: libuv/libuv#4089

Author: bnoordhuis

src/unix/linux.c

@@ -431,8 +431,14 @@ static int uv__use_io_uring(void) {
   use = atomic_load_explicit(&use_io_uring, memory_order_relaxed);
 
   if (use == 0) {
+    /* Older kernels have a bug where the sqpoll thread uses 100% CPU. */
+    use = uv__kernel_version() >= /* 5.10.186 */ 0x050ABA ? 1 : -1;
+
+    /* But users can still enable it if they so desire. */
     val = getenv("UV_USE_IO_URING");
-    use = val == NULL || atoi(val) ? 1 : -1;
+    if (val != NULL)
+      use = atoi(val) ? 1 : -1;
+
     atomic_store_explicit(&use_io_uring, use, memory_order_relaxed);
   }