pimd: Fix missing pimreg interface

pimd: Fix missing pimreg interface

`pimregX` of specific vrf can be deleted from kernel after this vrf
is deleted.  However, then `pimdregX` will never come back to
kernel after adding it ( the same vrf ) back.  That is to say, it exists
only in daemon, but not in kernel.

The root cause is this `pimregX` is not really deleted for its special
usage, and `pim_if_create_pimreg()` wants reusing it.

I have tried 4 solutions:
1. Remove the `configured` flag of `pimregX`, allow its deletion.
A few timers still use `pimregX` after its deletion.  So, not adopted.
2. Remove `pimregX` by vrf hook in `pim_instance_terminate()`.
It has no vrf id there.  So, not adopted.
3. Reuse `pimregX` in `pim_if_create_pimreg()`.
If `pim->regiface->info` is true, then reuse old `pimregX` and only call
`pim_if_add_vif()` to install it into kernel.  But at that time, it maybe
is in default VRF.  The `pim_zebra_interface_set_master()` doesn't work
at that time because it shouldn't wait there for its changing into
correct VRF.  So, not adopted.
4. Not reuse it.
If `pim->regiface->info` is true, there must have been pim instance with
VRF deleted and created before.  Actually delele old one in
`pim_if_create_pimreg()`, then recreate new one.

Finally, this PR adopted the fourth solution.

Fixes #13454

Signed-off-by: anlan_cs <vic.lan@pica8.com>