nbd: support NBD_SET_FLAGS ioctl

The nbd kernel module cannot enable DISCARD requests unless it is
informed about it.  The flags field in the header is used for this,
and this patch adds support for it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

nbd.c

@@ -378,6 +378,14 @@ int nbd_init(int fd, int csock, uint32_t flags, off_t size, size_t blocksize)
         }
     }
 
+    if (ioctl(fd, NBD_SET_FLAGS, flags) < 0
+        && errno != ENOTTY) {
+        int serrno = errno;
+        LOG("Failed setting flags");
+        errno = serrno;
+        return -1;
+    }
+
     TRACE("Clearing NBD socket");
 
     if (ioctl(fd, NBD_CLEAR_SOCK) == -1) {