if_bridge: Mask MEXTPG if some members don't support it

[markj]

Summary

Similar to how the network stack needs to use mb_unmapped_to_ext()
to convert mbufs before passing them to an unsupported driver, if_bridge
needs to avoid passing M_EXTPG mbufs to interfaces. Thus, clear
IFCAP_MEXTPG if any member interfaces don't handle unmapped mbufs.

PR: 278245
MFC after: 2 weeks

[gallatin]

Why do we want or need a hardcoded list? Why can't this function be more like lagg_capabilities()? If we do want a hardcoded list, what about IFCAP_TXTLS*

[markj]

In D47294#1078776, @gallatin wrote:

Why do we want or need a hardcoded list? Why can't this function be more like lagg_capabilities()? If we do want a hardcoded list, what about IFCAP_TXTLS*

That's a good question. if_bridge could probably be smarter, indeed.

Why exactly does if_bridge need to care about IFCAP_TXTLS*?

[gallatin]

In D47294#1078779, @markj wrote:

In D47294#1078776, @gallatin wrote:

Why do we want or need a hardcoded list? Why can't this function be more like lagg_capabilities()? If we do want a hardcoded list, what about IFCAP_TXTLS*

That's a good question. if_bridge could probably be smarter, indeed.

Why exactly does if_bridge need to care about IFCAP_TXTLS*?

The same reason lagg does.... if its enabled (along with MEXTPG) and the bridge interface is used as an egress interface, we'll at the very least attempt to alloc a send tag.. I'm not 100% certain it would be a problem, but it seems more obvious to mask the feature off