ceph: switch to page_mkwrite_check_truncate in ceph_page_mkwrite

Use the "page has been truncated" logic in page_mkwrite_check_truncate
instead of reimplementing it here.  Other than with the existing code,
fail with -EFAULT / VM_FAULT_NOPAGE when page_offset(page) == size here
as well, as should be expected.
Signed-off-by: NAndreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

fs/ceph/addr.c

@@ -1655,7 +1655,7 @@ static vm_fault_t ceph_page_mkwrite(struct vm_fault *vmf)
 	do {
 		lock_page(page);
 
-		if ((off > size) || (page->mapping != inode->i_mapping)) {
+		if (page_mkwrite_check_truncate(page, inode) < 0) {
 			unlock_page(page);
 			ret = VM_FAULT_NOPAGE;
 			break;