mm/memory-failure: fix an incorrect use of tail pages

When backport commit c79c5a0a to 4.19-stable, there is a mistake change.
The head page instead of tail page should be passed to try_to_unmap(),
otherwise unmap will failed as follows.

 Memory failure: 0x121c10: failed to unmap page (mapcount=1)
 Memory failure: 0x121c10: recovery action for unmapping failed page: Ignored

Fixes: c6f50413

 ("mm/memory-failure: check the mapcount of the precise page")
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>