module: Move module_assert_mutex_or_preempt() to internal.h

No functional change.

This patch migrates module_assert_mutex_or_preempt() to internal.h.
So, the aforementiond function can be used outside of main/or core
module code yet will remain restricted for internal use only.

Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>