hw/display/virtio-gpu: fix dmabuf_fd leak on remap failure

When virtio_gpu_create_udmabuf() succeeds but virtio_gpu_remap_udmabuf()
fails (mmap returns MAP_FAILED), virtio_gpu_init_udmabuf() returns early
without closing the dmabuf fd. Since res->blob is never set in this
path, later cleanup via virtio_gpu_cleanup_mapping() skips
virtio_gpu_fini_udmabuf() entirely, leaking the file descriptor.

Call virtio_gpu_destroy_udmabuf() before the early return to close
the fd. This function already handles partial state correctly: it
skips the munmap when res->remapped is NULL and closes the fd when
res->dmabuf_fd >= 0.

Fixes: 9b60cdf98723 ("virtio-gpu: Add udmabuf helpers")
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Message-ID: <20260713125622.111513-1-marcandre.lureau@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
(cherry picked from commit 564c4d7e99d87b39c3faea52ef8759df7f786f61)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>