ANDROID: arm64: Fix MMIO guard ioremap when called before slab_is_available().

The ioremap hook can be called before slab is initialised, at which time
calling into kmalloc() is not allowed.

Signed-off-by: Keir Fraser <keirf@google.com>
Bug: 232894028
Fixes: f89d2055 ("ANDROID: arm64: Implement ioremap/iounmap hooks calling into KVM's MMIO guard")
Change-Id: Ieaf5adbdacdb196e37f4629998164a015e15c6d8