net: if: do not remove the solicited-node multicast address if used

Two different IPv6 addresses can have the same solicited node multicast
address, for example when they are derived from the same EUI-64
interface identifier during the auto-configuration process. For example,
an interface with a physical address 70:07:12:34:56:78 can have the
following:
- link-local address FE80::7207:12FF:FE34:5678
- global unicast address 2001:1234:::7207:12FF:FE34:5678

Both addresses will have the same solicited-node multicast address
FF02::1:FF34:5678. Currently, if one removes the global unicast address,
the solicited-node multicast address is also removed, leaving the
link-local address out of the solicited-node multicast group. This
breaks some protocols like Neighbour Discovery.

Count how many times the solicited-node multicast address is used and
remove it only if it is not shared by any other unicast address.

Fixes #59683

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>