cifsd: Fix potential null-ptr-deref in smb2_open()

Fix potential null-ptr-deref in smb2_open().

Signed-off-by: Marios Makassikis <mmakassikis@freebox.fr>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>