fs/cifs: Fix resource leak

The -EIO error return path is leaking memory allocated
to page. Fix this by moving the allocation block after
the check of cifs_forced_shutdown.

Addresses-Coverity: ("Resource leak")
Fixes: 087f757b ("cifs: add shutdown support")
Signed-off-by: Khaled ROMDHANI <khaledromdhani216@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <stfrench@microsoft.com>