verity: remove the depth parameter from bht_create We want to only support regular tries with a single root hash block. BUG=chromium-os9752 TEST=Ran dm-verity.git unit tests. Ran platform_DMVerityCorruption on H/W. Change-Id: I49a8b74b8343c4cb5aa871a81b45f06025fe1011 R=wad@chromium.org,taysom@chromium.org,ups@chromium.org Review URL: http://codereview.chromium.org/6811030