net: ibm: emac: Reserve VLAN header in MJS limit

The IBM EMAC programs its Maximum Jumbo Size (MJS) drop
threshold from ndev->mtu directly. The hardware sizes the threshold
against the L2 frame minus the ethernet header, but does not
discount the 802.1Q tag, so a frame carrying a VLAN tag and a full
1500-byte payload exceeds MJS by exactly 4 bytes and is dropped.

This is normally hidden because JPSM (and therefore the MJS check)
only engages when the MTU is raised above ETH_DATA_LEN.  With the
qca8k DSA tagger the conduit MTU is bumped by QCA_HDR_LEN to 1502
during dsa_conduit_setup(), which is enough to enable JPSM and
expose the off-by-VLAN-tag in the limit.

Pad MJS by VLAN_HLEN so a VLAN-tagged full-MTU frame passes.

Reported on Meraki MX60 (qca8k switch): tagged VLAN
traffic drops at 1500-byte payload, while 1496 bytes works
and untagged 1500 bytes works.

Assisted-by: Claude:Opus-4.7
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260526202247.13823-1-rosenp@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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