ANDROID: Incremental fs: Don't allow renaming .index directory.

Add additional check to verify dentry is hashed after vfs_mkdir()
is successful.

Bug: 148423333
Test: kernel seftest - incfs_test, manual test removing .index when
mounted

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I7d2d71d480db4f02b7f3a347b5734c6643843594