ANDROID: Incremental fs: dentry_revalidate should not return -EBADF.

Fix of bug 161565969 added EBADF when backing path doesn't
exist. However, .log and .pending reads doesn't have
backing path causing incfs_test to fail.

Bug: 162243475
Test: incfs_test on QEMU passes

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Iba303bbc42b431fe4a4fbacb40584faaff4b14b7