eth: fbnic: don't build the driver when skb has more than 21 frags

eth: fbnic: don't build the driver when skb has more than 21 frags

Similarly to commit 0e03c643dc93 ("eth: fbnic: fix s390 build."),
the driver won't build if skb_shared_info has more than 25 frags
assuming a 64B cache line and 21 frags assuming a 128B cache line.

  (512 - 48 -  64) / 16 = 25
  (512 - 48 - 128) / 16 = 21

Fixes: 0cb4c0a13723 ("eth: fbnic: Implement Rx queue alloc/start/stop/free")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20240717161600.1291544-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>