uml: fix previous request size limit fix

The previous patch which limited the number of sectors in a single request
to a COWed device was correct in concept, but the limit was implemented in
the wrong place.

By putting it in ubd_add, it covered the cases where the COWing was
specified on the command line.  However, when the command line only has the
COW file specified, the fact that it's a COW file isn't known until it's
opened, so the limit is missed in these cases.

This patch moves the sector limit from ubd_add to ubd_open_dev.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>