net: mctp: put sock on tag allocation failure

[ Upstream commit 9990889b ]

We may hold an extra reference on a socket if a tag allocation fails: we
optimistically allocate the sk_key, and take a ref there, but do not
drop if we end up not using the allocated key.

Ensure we're dropping the sock on this failure by doing a proper unref
rather than directly kfree()ing.

Fixes: de8a6b15

 ("net: mctp: add an explicit reference from a mctp_sk_key to sock")
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/ce9b61e44d1cdae7797be0c5e3141baf582d23a0.1707983487.git.jk@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>