fix(common): Simplify unmount code, die loud and die hard.

As-is safe_umount is extremely dangerous. When passed multiple mount
points and any one of them fail with a "not mounted" or "doesn't exist"
error then any others that fail with a more serious error will be
silently ignored. This can cause untold sadness when running deleting a
chroot with cros_sdk if /mnt/host/source is left mounted, all your code
will be gone. To avoid this situation remove *ALL* this extra logic and
die very loudly when umount fails. Due to the way bind mounts interact
with this code "not mounted" so when unmounting a full tree we need to
still need to gracefully retry when the first umount fails.

common.sh

@@ -663,9 +663,9 @@ safe_umount_tree() {
     return 0
   fi
 
-  # First try to unmount in one shot to speed things up.
-  if safe_umount -d ${mounts}; then
-    return 0
+  # First try to unmount, this might fail because of nested binds.
+  if sudo umount -d ${mounts}; then
+    return 0;
   fi
 
   # Check whether our mounts were successfully unmounted.
@@ -675,45 +675,19 @@ safe_umount_tree() {
     return 0
   fi
 
-  # Well that didn't work, so lazy unmount remaining ones.
+  # Try one more time, this one will die hard if it fails.
   warn "Failed to unmount ${mounts}"
-  warn "Doing a lazy unmount"
-  if ! safe_umount -d -l ${mounts}; then
-    mounts=$(sub_mounts "$1")
-    die "Failed to lazily unmount ${mounts}"
-  fi
+  safe_umount -d ${mounts}
 }
 
 
 # Run umount as root.
 safe_umount() {
-  local ret=0
-  local out=""
-  set +e
-  for i in $(seq 1 4); do
-    out=`$([[ ${UID:-$(id -u)} != 0 ]] && echo sudo) umount "$@" 2>&1`
-    ret=$?
-    if [[ $ret -eq 0 ]]; then
-      set -e
-      return 0
-    fi
-    # Mount is not found
-    if [[ `expr index "${out}" "not found"` -ne "0" ]]; then
-      set -e
-      return 0
-    fi
-    # Mount is not mounted.
-    if [[ `expr index "${out}" "not mounted"` -ne "0" ]]; then
-      set -e
-      return 0
-    fi
-    sleep 1
-    # Mount is actually busy.
-    if [[ `expr index "${out}" "busy"` -ne "0" ]]; then
-      continue
-    fi
-  done
-  return $ret
+  if sudo umount "$@"; then
+    return 0;
+  else
+    failboat safe_umount
+  fi
 }
 
 get_git_id() {