FUSE-BPF: Comment on why backing_fd is not closed

It's not clear in the code why a file is opened, but not closed.
That's because the file is automatically closed in the kernel in the
FUSE-BPF code.
Add a comment which explains this.

Test: none
Bug: 214034634
Signed-off-by: Alessio Balsini <balsini@google.com>
Change-Id: Ia2fdd36ae3a572ac9289e2ab4987bd59805c4d7d
1 file changed