ANDROID: sdcardfs: switch to full_name_hash and qstr

Use the kernel's string hash function instead of rolling
our own. Additionally, save a bit of calculation by using
the qstr struct in place of strings.

Change-Id: I0bbeb5ec2a9233f40135ad632e6f22c30ffa95c1
Signed-off-by: Daniel Rosenberg <drosen@google.com>