ext4: small write refactor + block leak fix

[mcimerman]

Pull request includes 2 commits:

• ext4: write: sparse/unallocated block helper fcn:

  • put handling of sparse or unallocated blocks into a helper function
  • this small refactor will help with implementing multi-block writes
  • also while at it put static in ext4_instance_get() definition, as it should be, see the prototype

• ext4: write: fix block leak:

  • extends the helper function with allocated bool hint, based on which we can free
    the allocated block on later error paths, which previously didn't free it

I am putting this PR up earlier without waiting until I finish my multi-block read/write feature proposals, in hope that this will get merged faster as it's not a big change.

Feedback/improvements/edits are welcome.