block: add support for limiting gaps in SG lists

Another restriction inherited for NVMe - those devices don't support
SG lists that have "gaps" in them. Gaps refers to cases where the
previous SG entry doesn't end on a page boundary. For NVMe, all SG
entries must start at offset 0 (except the first) and end on a page
boundary (except the last).

Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/bio.c
Simple merge

diff --cc block/blk-merge.c
Simple merge

diff --cc include/linux/bio.h
Simple merge

diff --cc include/linux/blkdev.h
Simple merge