gtpu: Track the dst FIB entry instead of RR sourcing that

gtpu: Track the dst FIB entry instead of RR sourcing that

RR sourcing the destination FIB entry limits the number of tunnels
to 255 for a particular destination. This change removes this limit.

Type: fix

The patch is based on 1f50bf8fc57ebf78f9056185a342493be460a847
that introduced the FIB entry tracking but did not update
the gtpu plugin.

Signed-off-by: Miklos Tirpak <[email protected]>
Change-Id: I8a4a87382a6eb5120e2bb65b9bc3c446bbfdbd3b
(cherry picked from commit 75c72369186f6341a13374d2dd6e60ce3c7a88a6)