filedesc: fix up breakage introduced in 272505

Include sequence counter supports incoditionally [1]. This fixes reprted build
problems with e.g. nvidia driver due to missing opt_capsicum.h.

Replace fishy looking sizeof with offsetof. Make fde_seq the last member in
order to simplify calculations.

Suggested by:	kib [1]
X-MFC:		with 272505


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272567 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f