mm/truncate: Inline invalidate_complete_page() into its one caller

invalidate_inode_page() is the only caller of invalidate_complete_page()
and inlining it reveals that the first check is unnecessary (because we
hold the page locked, and we just retrieved the mapping from the page).
Actually, it does make a difference, in that tail pages no longer fail
at this check, so it's now possible to remove a tail page from a mapping.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>