VerityUtils: handle ENOTTY from FS_IOC_GETFLAGS

In some cases, such as EROFS filesystems, it's expected for
statxForFsverity() to get ENOTTY from FS_IOC_GETFLAGS.  In this case, we
should just return 0 to indicate that fs-verity is not enabled.  This
avoids logging an ERROR message to the log unnecessarily.

Bug: 279597861
Change-Id: I58149f7611c223789210594f28a98420dfddd8ec