ANDROID: Incremental fs: Fix minor bugs

1: Invoke kunmap(page) in error path
2: Validate NULL checks at few places in the code.
3: path_put() should not be invoked if path entry is null.
   Although path_put() checks for NULL condition internally,
   caller should gracefully handle it.

Bug: 161565969
Test: kernel selftest - incfs_test, incfs_perf

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: Ie4dfaaba4b09f4798d492f8a25dd9dcc8da89e51