block_invalidatepage(): only release page if the full page was invalidated

Prior to commit d47992f8 ("mm: change invalidatepage prototype to
accept length"), an offset of 0 meant that the full page was being
invalidated.  After that commit, we need to instead check the length.

Jan said:
:
: The only possible issue is that try_to_release_page() was called more
: often than necessary.  Otherwise the issue is harmless but still it's good
: to have this fixed.

Link: http://lkml.kernel.org/r/x49fu5rtnzs.fsf@segfault.boston.devel.redhat.com
Fixes: d47992f8

 ("mm: change invalidatepage prototype to accept length")
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Lukas Czerner <lczerner@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>