The generic netmap TX pool always allocates standard clusters
(zone_clust), even though the buffer size may be larger.

INVARIANTS kernels panic in nm_os_get_mbuf() on first transmit through a
non-native NIC (e.g. re or epair) using netmap with jumbo frames
configured. Release kernels silently truncate frames in
nm_os_generic_xmit_frame().

Implement cluster zone selection based on the requested size. Catch
oversized requests in nm_os_catch_tx() and return EINVAL. Add a KASSERT
before the truncation fallback in xmit.

Tested using epair, pkt-gen, and tcpdump on release (15.1) and
INVARIANTS (main) kernels at buffer sizes 2048/4096/9216/16384.