xfs: don't let memory failures leak blocks and kill repairs

In the Linux kernel, the following vulnerability has been resolved: xfs: don't let memory failures leak blocks and kill repairs LOLLM complains that a memory allocation failure in xrep_newbt_add_blocks results in online repair leaking blocks that were previously allocated to write a new btree, but the problem is worse than that -- a limitation of the codebase is that the callers cannot undo the transaction /and/ return the error -- either you undo all changes and commit the transaction, or you error out and the filesystem goes down. However, the new btree space reservation object isn't that big (~48 bytes). Let's just do a NOFAIL allocation and the problem goes away.