net: mctp i3c: Copy headers if cloned

Use skb_cow_head() prior to modifying the tx skb. This is necessary
when the skb has been cloned, to avoid modifying other shared clones.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Fixes: c8755b29 ("mctp i3c: MCTP I3C driver")
Link: https://patch.msgid.link/20250306-matt-i3c-cow-head-v1-1-d5e6a5495227@codeconstruct.com.au


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