blk: blkmap: Add linear device mapping support

Allow a slice of an existing block device to be mapped to a
blkmap. This means that filesystems that are not stored at exact
partition boundaries can be accessed by remapping a slice of the
existing device to a blkmap device.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Simon Glass <sjg@chromium.org>