dm: make EBS depend on !HIGHMEM

__ebs_rw_bvec use page_address on the submitted bios data, and thus
can't deal with highmem.  Disable the target on highmem configs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210804095634.460779-4-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>