filedesc: tidy up fdcopy a little bit

Test for file availability by fde_file != NULL instead of fdisused, this is
consistent with similar checks later.

Drop badfileops check. badfileops don't have DFLAG_PASSABLE set, so it was never
reached in practice.

fdiused is now only used in some KASSERTS, so ifdef it under INVARIANTS.

No functional changes.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273878 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f