MINOR: buffer: replace bo_getblk_nc() with b_getblk_nc() which takes an offset

MINOR: buffer: replace bo_getblk_nc() with b_getblk_nc() which takes an offset

This will be important so that we can parse a buffer without touching it.
Now we indicate where from the buffer's head we plan to start to copy, and
for how many bytes. This will be used by send functions to loop at the end
of the buffer without having to update the buffer's output byte count.