amt: make the head writable before rewriting the L2 header

amt: make the head writable before rewriting the L2 header

amt_multicast_data_handler(), amt_membership_query_handler() and
amt_update_handler() rewrite the ethernet header of the decapsulated skb
in place (eth->h_proto, eth->h_dest and, for the query, also
eth->h_source) before handing it up the stack.  The skb head may be
shared, for example when a packet tap has cloned it on the underlay
interface, so writing through it corrupts the other reader's copy.

Call skb_cow_head() before the rewrite so the head is private.  It is
placed before the pointers into the head are (re-)derived, so a
reallocation caused by the copy is picked up by those derivations.

Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface")
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Taehee Yoo <ap420073@gmail.com>
Link: https://patch.msgid.link/20260711151934.2955226-3-michael.bommarito@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>