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

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 that don't support
them. Thus, clear IFCAP_MEXTPG on the bridge if any member interfaces
don't handle unmapped mbufs.

Approved by: re (kib)
PR: 278245
Reviewed by: jhb, gallatin
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47294

(cherry picked from commit 2bbfbf80d3bb828ac782c2d990a1fba0eb51e45a)
(cherry picked from commit 01a3c17d18bafbf4e76e9d77e8862525b50602e9)