ANDROID: null_blk: Support configuring the maximum segment size

Add support for configuring the maximum segment size.

Add support for segments smaller than the page size.

This patch enables testing segments smaller than the page size with a
driver that does not call blk_rq_map_sg().

Bug: 308663717
Bug: 319125789
Change-Id: Idd7094e9f773c295017b44377d2a3f10abea95cf
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Juan Yescas <jyescas@google.com>