Release an open file
Release is called when there are no more references to an open file: all file descriptors are closed and all memory mappings are unmapped.
For every open call there will be exactly one release call.
The filesystem may reply with an error, but error values are not returned to close() or munmap() which triggered the release.
fi->fh will contain the value set by the open method, or will be undefined if the open method didn't set any value. fi->flags will contain the same flags as for open.
Valid replies: fuse_reply_err