lib: kunit_iov_iter: fix test fail on powerpc

lib: kunit_iov_iter: fix test fail on powerpc

Increase buffer size to accommodate machines with 64K PAGE_SIZE.

Link: https://lore.kernel.org/20260421070707.992873-1-lk@c--e.de
Fixes: 0913b7554726 ("lib: kunit_iov_iter: add tests for extract_iter_to_sg")
Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
Reported-by: David Gow <davidgow@google.com>
Closes: https://lore.kernel.org/34a81ec2-af84-465d-9b5e-7bb5bf01680f@davidgow.net
Tested-by: David Gow <davidgow@google.com>
Tested-by: Josh Law <joshlaw48@gmail.com>
Reviewed-by: Josh Law <joshlaw48@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>