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: I74165d83e71201116378c4598c2f9a2ff8c8b623
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Juan Yescas <jyescas@google.com>