ynl: fix schema check errors

ynl: fix schema check errors

Fix two schema check errors that have lurked since the attribute name
validation was made more strict:

not ok 2 conntrack.yaml schema validation
'labels mask' does not match '^[0-9a-z-]+$'

not ok 13 nftables.yaml schema validation
'set id' does not match '^[0-9a-z-]+$'

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20251127123502.89142-5-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>