zebra: Fix incorrect reading of REMOTE_VTEP_[ADD|DEL]

zebra: Fix incorrect reading of REMOTE_VTEP_[ADD|DEL]

With flooding control added recently we were not properly handling
the new flood control parameter in zebra_vxlan.c handler functions.
The error message that was being repeatedly seen:

2019/05/01 00:47:32 ZEBRA: [EC 100663311] stream_get2: Attempt to get out of bounds
2019/05/01 00:47:32 ZEBRA: [EC 100663311] &(struct stream): 0x7f0f04001740, size: 22, getp: 22, endp: 22

The fix was to ensure that both the _add and _del functions kept proper
sizing of amount of data read *and* the _del function was not
reading the flood_control data from the stream.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>