ANDROID: fuse-bpf: Fix revalidate error path and backing handling

Currently we have 2 different problems

1. Every revalidate considered as a error because of added
args->out_argvar = true; inside fuse_lookup_init which makes
fuse_simple_request return out argument size which is
considered as an error by revalidate code.

2. We’re ignoring backing_fd and bpf_program set by daemon
lookup code called by revalidate.

Problem 1 makes any revalidate (lookup to userspace) useless and any result
lead us to the full lookup because it was interpreted as an error.

This CL fixes both and introducing revalidate test case which makes sure:

1. We’re receiving only one lookup as a part of revalidate
2. We’re setting backing_fd as a part of revalidate’s lookup result

Test is failed before the fix and passed after.

Bug: 219958836
Test: Booted device 5 times to make sure we’re not receiving redundant
lookups anymore.
Test: selftests
Signed-off-by: Dmitrii Merkurev <dimorinny@google.com>
Change-Id: Ifa62e56b42ca5580b25682eb5f16b5c91826cf49