hostfs: Add a BUG_ON to detect behavior changes of dentry_path_raw()

hostfs' __dentry_name() relies on the fact that dentry_path_raw() will place
the path name at the end of the provided buffer.
While this is okay, add a BUG_ON() to detect behavior changes as soon
as possible.

Signed-off-by: Richard Weinberger <richard@nod.at>