MINOR: buffer: replace bo_getblk() with direction agnostic b_getblk()

MINOR: buffer: replace bo_getblk() with direction agnostic b_getblk()

This new functoin limits itself to the amount of data available in the
buffer and doesn't care about the direction anymore. It's only called
from co_getblk() which already checks that no more than the available
output bytes is requested.