bgpd: Fix memory leak in SRv6 locator delete

bgpd: Fix memory leak in SRv6 locator delete

Running `bgp_srv6l3vpn_to_bgp_vrf` and `bgp_srv6l3vpn_to_bgp_vrf2`
topotests with `--valgrind-memleaks` gives several memory leak errors.
This is due to the way SRv6 locators are removed/unset in bgpd: when
an SRv6 locator is deleted or unset, the memory allocated for the
locator prefix (`tovpn_sid_locator`) is not freed.

This patch adds a `for` loop that iterates over the list of BGP
instances. For each BGP instance using the SRv6 locator to be
removed/unset, we use `XFREE()` to properly free the memory allocated
for `tovpn_sid_locator` after the SRv6 locator is removed or unset.

The memory allocated for `tovpn_sid_locator` cannot be freed before
calling `vpn_leak_postchange_all()`. This is because
after deleting an SRv6 locator, we call `vpn_leak_postchange_all()`
to handle the SRv6 locator deletion and send a BGP Prefix SID withdraw
message. `tovpn_sid_locator` is required to properly build the BGP
Prefix SID withdraw message. After calling `vpn_leak_postchange_all()`
we can safely remove the `tovpn_sid_locator` and free the allocated
memory.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>