3.2.67-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Ben Hutchings <ben@decadent.org.uk>

We need to check the position and size of file writes against various
limits, using generic_write_check().  This was not being done for
the splice write path.  It was fixed upstream by commit 8d020765
("->splice_write() via ->write_iter()") but we can't apply that.

CVE-2014-7822
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Zefan Li <lizefan@huawei.com>