vxlan: bump and change to PKG_RELEASE

vxlan: bump and change to PKG_RELEASE

Bumping package version has been overlooked in a previous commit.

While at it, use PKG_RELEASE instead of PKG_VERSION, as the latter
is meant for upstream version number only.
(The effective version string for the package would be "3" in both
cases, so there is no harm done for version comparison.)

Fixes: 0453c3866feb ("vxlan: fix udp checksum control")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit b29d620ed2521fe6fda40ddafe6cb0f1d70e4503)