RDMA/core/iwpm_msg: Add proper descriptions for 'skb' param

RDMA/core/iwpm_msg: Add proper descriptions for 'skb' param

Fixes the following W=1 kernel build warning(s):

 drivers/infiniband/core/iwpm_msg.c:402: warning: Function parameter or member 'skb' not described in 'iwpm_register_pid_cb'
 drivers/infiniband/core/iwpm_msg.c:475: warning: Function parameter or member 'skb' not described in 'iwpm_add_mapping_cb'
 drivers/infiniband/core/iwpm_msg.c:553: warning: Function parameter or member 'skb' not described in 'iwpm_add_and_query_mapping_cb'
 drivers/infiniband/core/iwpm_msg.c:636: warning: Function parameter or member 'skb' not described in 'iwpm_remote_info_cb'
 drivers/infiniband/core/iwpm_msg.c:716: warning: Function parameter or member 'skb' not described in 'iwpm_mapping_info_cb'
 drivers/infiniband/core/iwpm_msg.c:773: warning: Function parameter or member 'skb' not described in 'iwpm_ack_mapping_info_cb'
 drivers/infiniband/core/iwpm_msg.c:803: warning: Function parameter or member 'skb' not described in 'iwpm_mapping_error_cb'
 drivers/infiniband/core/iwpm_msg.c:851: warning: Function parameter or member 'skb' not described in 'iwpm_hello_cb'

Link: https://lore.kernel.org/r/20210118223929.512175-21-lee.jones@linaro.org
Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>