Fix directory comparison in tryMountDataMirror

Inode numbers are only distinct within a single filesystem.  To
determine whether two paths name the same file, both the inode number
and device number need to be compared.

Also downgrade the log message to INFO since this case is expected.

Test: On Cuttlefish:
    sm partition disk:253,32 private
    logcat | grep 'is mounted already' # message still appears

Change-Id: Ic1db53d4c3842b3ecc57a498b8c1300377b27e8a