RDMA/core: Prefer NLA_NUL_STRING

RDMA/core: Prefer NLA_NUL_STRING

These attributes are evaluated as c-string (passed to strcmp), but
NLA_STRING doesn't check for the presence of a \0 terminator.

Either this needs to switch to nla_strcmp() and needs to adjust printf fmt
specifier to not use plain %s, or this needs to use NLA_NUL_STRING.

As the code has been this way for long time, it seems to me that userspace
does include the terminating nul, even tough its not enforced so far, and
thus NLA_NUL_STRING use is the simpler solution.

Fixes: 30dc5e63d6a5 ("RDMA/core: Add support for iWARP Port Mapper user space service")
Link: https://patch.msgid.link/r/20260330122742.13315-1-fw@strlen.de
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>