aoe: fix sysfs_create_file warnings

Moved the attributes into a group, making the compiler be quiet about
ignoring the return value of the file create calls.  This also also
fixed a bug when removing the files, which were not symlinks.

Cc: "Ed L. Cashin" <ecashin@coraid.com>
Cc: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>