IB/cm: Split cm_alloc_msg()

This is being used with two quite different flows, one attaches the
message to the priv and the other does not.

Ensure the message attach is consistently done under the spinlock and
ensure that the free on error always detaches the message from the
cm_id_priv, also always under lock.

This makes read/write to the cm_id_priv->msg consistently locked and
consistently NULL'd when the message is freed, even in all error paths.

Link: https://lore.kernel.org/r/f692b8c89eecb34fd82244f317e478bea6c97688.1622629024.git.leonro@nvidia.com
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>