adb: Correctly detect deduplicated ext4 partitions.

The fs_has_shared_blocks function had two problems. One, it called
statfs() on the mount point, which will return information about the
file system the mount point is on, rather than the filesystem mounted at
that location. Second, the check for EXT4_SUPER_MAGIC was inverted.

Bug: N/A
Test: adb remount -R works on an ext4 deduplicated device
Change-Id: I2e5ef895ea274cb7cc3c35295120da90a026d0d2