constify blk_rq_map_user_iov() and friends

sg_iovec array passed to it can be const

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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

diff --cc fs/bio.c
Simple merge

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

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