splice: Apply generic position and size checks to each write

splice: Apply generic position and size checks to each write

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>
[ kamal: port to 3.13-stable: context ]
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>