Currently bread()-ing through device vnode with
(1) VMIO enabled,
(2) bo_bsize != DEV_BSIZE
(3) more than 1 block
results in data being incorrectly cached.
So instead a more common approach of using a vnode belonging to fs is now
employed.
Also, prevent attempt to bread more than MAXBSIZE bytes because of
adjustments made to account for offset that doesn't start on block
boundary.
Add expanded comments to explain the calculations.
Also drop unused inline function while here.
PR: kern/120967
PR: kern/129084
Reviewed by: scottl, kib
Approved by: jhb (mentor)
b0c0fb5940