Simplify and fix block allocation

This was just totally broken.  Since we used to re-alloc the pes blocks,
we cannot maintain a list of them: the pointers change.

So stop realloc'ing the blocks, and instead just realloc the stitch
list.  And simplify the list handling while at it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>