sdcardfs: Change magic value

Sdcardfs uses the same magic value as wrapfs.
This should not be the case. As it is entirely
in memory, the value can be changed without any
loss of compatibility.

Change-Id: I24200b805d5e6d32702638be99e47d50d7f2f746
Signed-off-by: Daniel Rosenberg <drosen@google.com>