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 8d0207652cbe
("->splice_write() via ->write_iter()") but we can't apply that.
CVE-2014-7822
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>