psample: adjust size if rate_as_probability is set

If PSAMPLE_ATTR_SAMPLE_PROBABILITY flag is to be sent, the available
size for the packet data has to be adjusted accordingly.

Also, check the error code returned by nla_put_flag.

Fixes: 7b1b2b60 ("net: psample: allow using rate as probability")
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20241217113739.3929300-1-amorenoz@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>