ANDROID: Incremental fs: Make fill block an ioctl

Filling blocks is not equivalent to writing a file, since they are
constrained by the root hash. selinux policy may wish to treat them
differently, for instance.

Test: incfs_test passes
Bug: 138149732
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: Ic369b84b92547b1cfefe422bd881c4e466090aed