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: 261097222
Change-Id: I365a7ce31066177e8c5ad1176fefaf1dde6d3173
Signed-off-by: Bart Van Assche <bvanassche@google.com>