um: increase default virtual physical memory to 64 MiB

The current 32 MiB of RAM causes OOMs to appear shortly after
booting in a minimal OpenWrt 22.03 configuration with a
5.10.134 kernel.

Of course, passing a "mem=64M" (from the --help text) parameter
works too, but it produces the following (info) message:

| [    0.000000] Unknown kernel command line parameters "mem=64M", will be passed to user space.

That's why, I think it would be nicer, if this is working out
of the box again :).

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>