net/smc: fix invalid return code in smcd_new_buf_create()

smc_ism_register_dmb() returns error codes set by the ISM driver which
are not guaranteed to be negative or in the errno range. Such values
would not be handled by ERR_PTR() and finally the return code will be
used as a memory address.
Fix that by using a valid negative errno value with ERR_PTR().

Fixes: 72b7f6c4

 ("net/smc: unique reason code for exceeded max dmb count")
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>