ANDROID: fuse-bpf: Fix truncate

Maps would crash every second launch. This was caused by maps receiving
an incorrect file size after truncate, then mapping the file.

Bug: 215486645
Test: fuse_test + launch maps 100 times & look for native crashes
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I13b0211330fb48592864ab53f8ffff60c19aab11