fuse: prepare for failing open response

In preparation for inode io modes, a server open response could fail due to
conflicting inode io modes.

Allow returning an error from fuse_finish_open() and handle the error in
the callers.

fuse_finish_open() is used as the callback of finish_open(), so that
FMODE_OPENED will not be set if fuse_finish_open() fails.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>