ANDROID: fuse-bpf: Always call revalidate for backing

If we have a backing dentry, we must call it's revalidate always, or we
may end up using an invalid lower dentry. Revalidate is called for
dentries, not inodes, and the dentry may be negative. This goes against
all of the macro conventions, so we're just calling the backing function
directly.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 219958836
Test: fuse_test
Change-Id: Ia28da5bd2ce42b40466c577137d5440d9f2f6600