netmap: Fix a queue length check in the generic port rx path

The check is ok by default, since the default value of
netmap_generic_ringsize is 1024. But we should check against the
configured "ring" size.

Reviewed by: vmaffione
MFC after: 1 week
Sponsored by: Zenarmor
Sponsored by: OPNsense
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38062

(cherry picked from commit 539437c8281d24450bacded3734276acd9983e90)