um: virt-pci: don't use kmalloc()

This code can be called deep in the IRQ handling, for
example, and then cannot normally use kmalloc(). Have
its own pre-allocated memory and use from there instead
so this doesn't occur. Only in the (very rare) case of
memcpy_toio() we'd still need to allocate memory.

Link: https://patch.msgid.link/20250110125550.32479-6-johannes@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>