os/bluestore/BlueFS: sanity check that alloc->allocate() won't return 0

os/bluestore/BlueFS: sanity check that alloc->allocate() won't return 0

If alloc[id]->allocate() returns 0, no space is actually reclaimed.
But BlueStore treats returning 0 as success and will keep reclaiming
space from BlueFS until hit the given threshold, which turns out to
be deadloop-prone.

For now, Bitmap and Stupid never return 0-length allocated space
(which is instead converted to -ENOSPC to suggest an error to caller),
so sanity checking against result code 0 will suffice.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>