MB-9307: Make rollback/2 an internal function call

`couch_set_view_updater:rollback/2` won't be called from the
outside, but just within the group.

"make check" in couch_set_view is expected to fail, it
will work again after the next commit.

Change-Id: I33cf920eb6293c0647a09b424ed082dd54f99802
Reviewed-on: http://review.couchbase.org/31300
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>