mm/truncate: Combine invalidate_mapping_pagevec() and __invalidate_mapping_pages()

We can save a function call by combining these two functions, which
are identical except for the return value.  Also move the prototype
to mm/internal.h.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>