cuse: fix ioctl

cuse_process_init_reply() doesn't initialize fc->max_pages and thus all
cuse bases ioctls fail with ENOMEM.

Reported-by: Andreas Steinmetz <ast@domdv.de>
Fixes: 5da784cc ("fuse: add max_pages to init_out")
Cc: <stable@vger.kernel.org> # v4.20
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>