fuse: zero initialize inode private data

This is slightly tricky, since the VFS uses non-zeroing allocation to
preserve some fields that are left in a consistent state.

Reported-by: Chunsheng Luo <luochunsheng@ustc.edu>
Closes: https://lore.kernel.org/all/20250818083224.229-1-luochunsheng@ustc.edu/


Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>