Fix a couple of problems which were introduced by a recent race fix in the
ext2 block allocator:

- if the allocation attempt raced, and lost the race then a new attempt is
  made.  But the earlier reservation must be put back first.

  Add a call to group_release_blocks() to fix this.

- if the filesystem is genuinely corrupted then the code as-is can get
  stuck in an infinite loop, thinking that a blockgroup has free blocks and
  then discovering that its bitmap is full.

  Fix this by baling out after having scanned all blockgroups twice.

  (Thanks Muli Ben-Yehuda <mulix@mulix.org> for spotting this).