fs/ntfs: Track the end of the MFT attribute buffer

fs/ntfs: Track the end of the MFT attribute buffer

The end of the attribute buffer should be stored alongside the rest of
the attribute struct as right now it is not possible to implement bounds
checking when accessing attributes sequentially.

This is done via:
  - updating init_attr() to set at->end and check is is not initially out of bounds,
  - implementing checks as init_attr() had its type change in its callers,
  - updating the value of at->end when needed.

Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>