This include was added by commit 187f1882b5b0 ("BUG: headers with
BUG/BUG_ON etc. need linux/bug.h") because BUG_ON() was used in this
header at that time.
Some time later, commit 6d75f366b924 ("lib: radix-tree: check accounting
of existing slot replacement users") removed the use of BUG_ON() from
this header.
Since then, there is no reason to include <linux/bug.h>.