• If caller specifies readbehind and readahead that together with count doesn't fit into a buf, then trim readbehind and readahead evenly. If rbehind was limited by the previous BMAP, then roundup its trim to block size.
• Add KASSERT to check that b_blkno has proper offset from original blkno returned by BMAP. [1]
• Add KASSERT to check that pages in buf are consecutive.

Reviewed by: kib
Submitted by: kib [1]