kunit: test: Export kunit_attach_mm()

kunit: test: Export kunit_attach_mm()

Tests can allocate from virtual memory using kunit_vm_mmap(), which
transparently creates and attaches an mm_struct to the test runner if
one is not already attached. This is suitable for most cases, except for
when the code under test must access a task's mm before performing an
mmap. Expose kunit_attach_mm() as part of the interface for those
cases. This does not change the existing behavior.

Cc: David Gow <davidgow@google.com>
Signed-off-by: Tiffany Yang <ynaffit@google.com>
Reviewed-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20250714185321.2417234-4-ynaffit@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>