Add FS_IOC_ENABLE_VERITY ioctl
When called, calculate measurement, validate signature against fsverity,
and set S_VERITY flag.
This does not (yet) preserve the verity status once the inode is
evicted.
Bug: 160634504
Test: incfs_test passes
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I88af2721f650098accc72a64528c7d85b753c7f6