MINOR: buffer: make bo_getblk_nc() not return 2 for a full buffer

MINOR: buffer: make bo_getblk_nc() not return 2 for a full buffer

Thus function returns the number of blocks. When a buffer is full and
properly aligned, buf->p loops back the beginning, and the test in the
code doesn't cover that specific case, so it returns two chunks, a full
one and an empty one. It's harmless but can sometimes have a small impact
on performance and definitely makes the code hard to debug.