mm/truncate.c: fix THP handling in invalidate_mapping_pages()

mm/truncate.c: fix THP handling in invalidate_mapping_pages()

The condition checking for THP straddling end of invalidated range is
wrong - it checks 'index' against 'end' but 'index' has been already
advanced to point to the end of THP and thus the condition can never be
true.  As a result THP straddling 'end' has been fully invalidated.
Given the nature of invalidate_mapping_pages(), this could be only
performance issue.  In fact, we are lucky the condition is wrong because
if it was ever true, we'd leave locked page behind.

Fix the condition checking for THP straddling 'end' and also properly
unlock the page.  Also update the comment before the condition to
explain why we decide not to invalidate the page as it was not clear to
me and I had to ask Kirill.

Link: http://lkml.kernel.org/r/20170619124723.21656-1-jack@suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>