vxlan: Copy needed_tailroom from lowerdev

[ Upstream commit a5e74021e84bb5eadf760aaf2c583304f02269be ]

While vxlan doesn't need any extra tailroom, the lowerdev might need it. In
that case, copy it over to reduce the chance for additional (re)allocations
in the transmit path.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Link: https://lore.kernel.org/r/20201126125247.1047977-2-sven@narfation.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>