virtiofs: add filesystem context source name check

In certain scenarios, for example, during fuzz testing, the source
name may be NULL, which could lead to a kernel panic. Therefore, an
extra check for the source name should be added.

Fixes: a62a8ef9 ("virtio-fs: add virtiofs filesystem")
Cc: <stable@vger.kernel.org> # all LTS kernels
Signed-off-by: Xiangsheng Hou <xiangsheng.hou@mediatek.com>
Link: https://lore.kernel.org/20250407115111.25535-1-xiangsheng.hou@mediatek.com


Signed-off-by: Christian Brauner <brauner@kernel.org>