system/physmem: use return value of ram_block_discard_require() as errno

When ram_block_discard_require() fails, errno is passed to error_setg_errno().
It's a stale value or 0 which is unrelated to ram_block_discard_require().

As ram_block_discard_require() already returns -EBUSY in failure case,
use it as errno for error_setg_errno().

Fixes: 852f0048f3ea ("make guest_memfd require uncoordinated discard")
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-ID: <20240716064213.290696-1-zhenzhong.duan@intel.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

system/physmem.c

@@ -1845,11 +1845,14 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
     }
 
     if (new_block->flags & RAM_GUEST_MEMFD) {
+        int ret;
+
         assert(kvm_enabled());
         assert(new_block->guest_memfd < 0);
 
-        if (ram_block_discard_require(true) < 0) {
-            error_setg_errno(errp, errno,
+        ret = ram_block_discard_require(true);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret,
                              "cannot set up private guest memory: discard currently blocked");
             error_append_hint(errp, "Are you using assigned devices?\n");
             goto out_free;