fuse: misc cleanups

* fuse_file_alloc() was structured in weird way.  The success path was
  split between else block and code following the block.  Restructure
  the code such that it's easier to read and modify.

* Unindent success path of fuse_release_common() to ease future
  changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>