net: mctp i2c: 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: f5b8abf9 ("mctp i2c: MCTP I2C binding driver")
Link: https://patch.msgid.link/20250306-matt-mctp-i2c-cow-v1-1-293827212681@codeconstruct.com.au


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