vxlan: vxlan_rcv(): Extract vxlan_hdr(skb) to a named variable

Having a named reference to the VXLAN header is more handy than having to
conjure it anew through vxlan_hdr() on every use. Add a new variable and
convert several open-coded sites.

Additionally, convert one "unparsed" use to the new variable as well. Thus
the only "unparsed" uses that remain are the flag-clearing and the header
validity check at the end.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/2a0a940e883c435a0fdbcdc1d03c4858957ad00e.1733412063.git.petrm@nvidia.com


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