RDMA/cma: Handle global/non-linklocal IPv6 addresses in cma_check_linklocal()

If addr is not a linklocal address, the code incorrectly fails to
return and ends up assigning the scope ID to the scope id of the
address, which is wrong.  Fix by checking if it's a link local address
first, and immediately return 0 if not.

Signed-off-by: Somnath Kotur <somnath.kotur@emulex.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>