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

Change-Id: I7c5fb5ee3d04a81d9cd7dc824e4b521103329f19
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>