gbp: Ownership of dynamically created vxlan-gbp tunnels managed via gbp_itf

gbp: Ownership of dynamically created vxlan-gbp tunnels managed via gbp_itf

Type: fix

This solves the ownership of vxlan-gbp tunnels. When the last reference of these goes away they need to be deleted. Currently there are two owners; gbp_itf via gef_itf and the lock held by the gbp_endpoint_location_t. The problem is that the
loc removes its reference whilst the fwd still holds the gbp_itf, and things go wrong.
This change moves the lifecycle management of the vxlan-gbp tunnel to the gbp_itf. When the last lock of the gbp_itf goes, so does the tunnel. now both the EP's loc and fwd can hold a lock on the gbp_itf and it's only removed when required.
The other change is the management of the 'user' of the gbp_itf. Since each user can enable and disable different features, it's the job of the gbp_itf to apply the combined set. determining a unique 'uesr' from the caller was near impossible, so I moved that to the gbp_itf, and return the allocated user, hence the 'handle' that encodes both user and interface.

The hash table maps from sw_if_index to pool index.

Change-Id: I4c7bf4c0e5dcf33d1c545f262365e69151febcf4
Signed-off-by: Neale Ranns <nranns@cisco.com>