lib: kunit_iov_iter: fix memory leaks

lib: kunit_iov_iter: fix memory leaks

Use vfree() instead of vunmap() to free the buffer allocated by
iov_kunit_create_buffer() because vunmap() does not honour
VM_MAP_PUT_PAGES.  In order for this to work the page array itself must
not be managed by kunit.

Remove the folio_put() when destroying a folioq.  This is handled by
vfree(), now.

Pointed out by sashiko.dev on a previous iteration of this series.

Tested by running the kunit test 10000 times in a loop.

Link: https://lkml.kernel.org/r/20260326214905.818170-4-lk@c--e.de
Fixes: 2d71340ff1d4 ("iov_iter: Kunit tests for copying to/from an iterator")
Signed-off-by: Christian A. Ehrhardt <lk@c--e.de>
Cc: David Howells <dhowells@redhat.com>
Cc: David Gow <davidgow@google.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>