ANDROID: KVM: arm64: Always keep a spare block for GFP_ATOMIC case

Donations from the CMA regions help to mitigate the host stage-2
fragmentation. However, it is not possible to allocate from that pool in
an atomic context. Always pre-alloc a spare block when possible so
GFP_ATOMIC allocations also have a chance to donate a block.

Bug: 383901011
Bug: 277989609
Bug: 357781595
Change-Id: I238d7e4c8d503a65b5f1e9d01cec98d2c59dc948
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>