Remove block_device:blk_file access from fsck.

Now that we have assigned specific types to userdata and cache
block devices, we can remove the ability of fsck to run on other
block devices.

Change-Id: I8cfb3dc0e4ebe6b73346ff291ecb11397bb0c2d0
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>