xfs: a couple getbmap cleanups

 - reshuffle various conditionals for data vs attr fork to make the code
   more readable
 - do fine-grainded goto-based error handling
 - exit early from conditionals instead of keeping a long else branch around
 - allow kmem_alloc to fail

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>