net/smc: fix suppressed return code

The patch that repaired the invalid return code in smcd_new_buf_create()
missed to take care of errno ENOSPC which has a special meaning that no
more DMBEs can be registered on the device. Fix that by keeping this
errno value during the translation of the return code.

Fixes: 6b1bbf94 ("net/smc: fix invalid return code in smcd_new_buf_create()")
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>