block/nbd: Fix missing ERRP_GUARD() for error_prepend()

As the comment in qapi/error, passing @errp to error_prepend() requires
ERRP_GUARD():

* = Why, when and how to use ERRP_GUARD() =
*
* Without ERRP_GUARD(), use of the @errp parameter is restricted:
...
* - It should not be passed to error_prepend(), error_vprepend() or
*   error_append_hint(), because that doesn't work with &error_fatal.
* ERRP_GUARD() lifts these restrictions.
*
* To use ERRP_GUARD(), add it right at the beginning of the function.
* @errp can then be used without worrying about the argument being
* NULL or &error_fatal.

ERRP_GUARD() could avoid the case when @errp is &error_fatal, the user
can't see this additional information, because exit() happens in
error_setg earlier than information is added [1].

The nbd_co_do_receive_one_chunk() passes @errp to error_prepend()
without ERRP_GUARD(), and though its @errp parameter points to its
caller's local_err, to follow the requirement of @errp, add missing
ERRP_GUARD() at the beginning of this function.

[1]: Issue description in the commit message of commit ae7c80a7bd
     ("error: New macro ERRP_GUARD()").

Cc: Eric Blake <eblake@redhat.com>
Cc: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20240311033822.3142585-6-zhao1.liu@linux.intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

block/nbd.c

@@ -852,6 +852,7 @@ static coroutine_fn int nbd_co_do_receive_one_chunk(
         BDRVNBDState *s, uint64_t cookie, bool only_structured,
         int *request_ret, QEMUIOVector *qiov, void **payload, Error **errp)
 {
+    ERRP_GUARD();
     int ret;
     int i = COOKIE_TO_INDEX(cookie);
     void *local_payload = NULL;