drm/vmwgfx: fix potential UAF in vmwgfx_surface.c

drm_file.master should be protected by either drm_device.master_mutex
or drm_file.master_lookup_lock when being dereferenced. However,
drm_master_get is called on unprotected file_priv->master pointers in
vmw_surface_define_ioctl and vmw_gb_surface_define_internal.

This is fixed by replacing drm_master_get with drm_file_get_master.

Signed-off-by: Desmond Cheong Zhi Xi <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Reviewed-by: Zack Rusin <[e-mail]>
Signed-off-by: Zack Rusin <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/vmwgfx/vmwgfx_surface.c

···
 	user_srf->prime.base.shareable = false;
 	user_srf->prime.base.tfile = NULL;
 	if (drm_is_primary_client(file_priv))
-		user_srf->master = drm_master_get(file_priv->master);
+		user_srf->master = drm_file_get_master(file_priv);
 
 	/**
 	 * From this point, the generic resource management functions
···
 
 	user_srf = container_of(srf, struct vmw_user_surface, srf);
 	if (drm_is_primary_client(file_priv))
-		user_srf->master = drm_master_get(file_priv->master);
+		user_srf->master = drm_file_get_master(file_priv);
 
 	res = &user_srf->srf.res;