Do not mount devices with invalid verity metadata

The return value of read_verity_metadata is propagated to caller
even if the verity metadata is invalid, provided that it can be
read from the device. This results in devices with invalid verity
metadata signatures to be mounted normally, which is not desirable.
This change fixes the bug by changing the return value in case of
verification failure to FS_MGR_SETUP_VERITY_FAIL.

Change-Id: Ic29f37a23cb417c2538d60fb05de9dd310d50f4a