mnt_idmapping: move ima-only helpers to ima

The vfs{g,u}id_{gt,lt}_* helpers are currently not needed outside of
ima and we shouldn't incentivize people to use them by placing them into
the header. Let's just define them locally in the one file in ima where
they are used.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>