ANDROID: sdcardfs: Get the blocksize from the lower fs

This changes sdcardfs to be more in line with the
getattr in wrapfs, which calls the lower fs's getattr
to get the block size

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Bug: 34723223
Change-Id: I1c9e16604ba580a8cdefa17f02dcc489d7351aed