ANDROID: fuse-bpf: Ensure bpf field can never be nulled

By putting and nulling fuse_inode's bpf field in fuse_evict_inode, we
left a race condition - this inode can still be active. Do not put the
bpf program until we are doing the final free in fuse_free_inode. This
was the root cause of the reported bug.

The backing inode cannot be put in fuse_free_inode, since put_inode can
sleep and this is called from an RCU handler. But the backing inode
cannot be freed until an RCU interval, so move the put_inode to the same
location as in overlayfs, which is destroy_inode.

Remove a path in fuse_handle_bpf_prog whereby bpf can be nulled out.
When we want to be able to null/change the bpf_prog in the future, we
will have to use a mutex or maybe RCU to protect existing users. But
until this time, ban this path.

Bug: 284450048
Test: fuse_test passes, Pixel 6 passes basic tests
Change-Id: Ie6844242f279a5b202eb021eac5a2dd3d08bf09d
Signed-off-by: Paul Lawrence <paullawrence@google.com>