module: unexport find_module and module_mutex

find_module is not used by modular code any more, and random driver code
has no business calling it to start with.

Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jessica Yu <jeyu@kernel.org>