Reading block devices to high addresses requires bounce buffer in low memory.

[xypron]

@NingYu00 @xiangxu-eswin
Merge request #19 is good enough for EFI applications reading into high memory to not fail. But we should try to understand which block device driver has a problem with high addresses.

For SATA this should already be addressed by commit 2122573 ("ata: eswin: allow r/w with high memory buffers").