ksmbd: fix NULL pointer dereference in smb2_get_info_filesystem()

If share is , share->path is NULL and it cause NULL pointer
dereference issue.

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-20479
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>