bufferlist: pre-allocate buffer array in one go with the buffer list

bufferlist: pre-allocate buffer array in one go with the buffer list

We can now create and free a buffer list with one slice alloc/free
call in most cases, instead of one slice alloc/free for the list,
one slice alloc/free for the GArray, and one malloc/free for the
GArray array. In practice we know the max size of our buffer list
from the start, so can avoid reallocs.

https://bugzilla.gnome.org/show_bug.cgi?id=732284