fuse: dax: set fc->dax to NULL in fuse_dax_conn_free()

fuse: dax: set fc->dax to NULL in fuse_dax_conn_free()

fuse_dax_conn_free() will be called when fuse_fill_super_common() fails
after fuse_dax_conn_alloc(). Then deactivate_locked_super() in
virtio_fs_get_tree() will call virtio_kill_sb() to release the discarded
superblock. This will call fuse_dax_conn_free() again in fuse_conn_put(),
resulting in a possible double free.

Fixes: 1dd539577c42 ("virtiofs: add a mount option to enable dax")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Cc: <stable@vger.kernel.org> # v5.10
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>