cachefiles: defer exposing anon_fd until after copy_to_user() succeeds

In the Linux kernel, the following vulnerability has been resolved:

cachefiles: defer exposing anon_fd until after copy_to_user() succeeds

After installing the anonymous fd, we can now see it in userland and close

it. However, at this point we may not have gotten the reference count of

the cache, but we will put it during colse fd, so this may cause a cache

UAF.

So grab the cache reference count before fd_install(). In addition, by

kernel convention, fd is taken over by the user land after fd_install(),

and the kernel should not call close_fd() after that, i.e., it should call

fd_install() after everything is ready, thus fd_install() is called after

copy_to_user() succeeds.