ANDROID: fuse-bpf: Partial fix for mmap'd files

This will simply pass all mapping operations to the backing file if it
exists. This is sufficient for our needs in Android13, but must be
extended in the future.

Signed-off-by: Paul Lawrence <paullawrence@google.com>
Bug: 202785178
Test: fuse_test passes
Change-Id: I3c51ef62415633ff0db039f25bfed1adf14e1e80