net/smc: Remove error handling of unregister_dmb()

net/smc: Remove error handling of unregister_dmb()

smcd_buf_free() calls smc_ism_unregister_dmb(lgr->smcd, buf_desc) and
then unconditionally frees buf_desc.

Remove the cleaning up of fields of buf_desc in
smc_ism_unregister_dmb(), because it is not helpful.

This removes the only usage of ISM_ERROR from the smc module. So move it
to drivers/s390/net/ism.h.

Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Mahanta Jambigi <mjambigi@linux.ibm.com>
Reviewed-by: Dust Li <dust.li@linux.alibaba.com>
Link: https://patch.msgid.link/20250918110500.1731261-2-wintera@linux.ibm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>