fuse: separate out fuse_conn_init() from new_conn()

Separate out fuse_conn_init() from new_conn() and while at it
initialize fuse_conn->entry during conn initialization.

This will be used by CUSE.

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