ANDROID: Incremental fs: Add FS_IOC_ENABLE_VERITY

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: I2d9c9cd9235fd3e14e3fd2ee191c26aafb6472d0