ANDROID: sdcardfs: Use filesystem specific hash

We weren't accounting for FS specific hash functions,
causing us to miss negative dentries for any FS that
had one.

Similar to a patch from esdfs
commit 75bd25a9476d ("esdfs: support lower's own hash")

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Change-Id: I32d1ba304d728e0ca2648cacfb4c2e441ae63608