bgpd: Set 4096 instead of 65535 as new max packet size for a new peer

bgpd: Set 4096 instead of 65535 as new max packet size for a new peer

New peers should be initialized with a usual max packet size and later
determined on OPEN messages.

Testing with different peers supporting/not supporting extended support.

2021/07/02 13:48:00 BGP: [WEV7K-2GAQ5] u2:s2 send UPDATE len 8991 (max message len: 65535) numpfx 1788
2021/07/02 13:48:03 BGP: [WEV7K-2GAQ5] u3:s3 send UPDATE len 4096 (max message len: 4096) numpfx 809
2021/07/02 13:48:03 BGP: [WEV7K-2GAQ5] u3:s3 send UPDATE len 4096 (max message len: 4096) numpfx 809

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>