geom: sort includes for some files

This is not exhaustive, just done ahead of some upcoming changes to
these files.

Don't include sys/cdefs.h explicitly. No functional change intended.

Reviewed by: imp, jhb
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42335

(cherry picked from commit f3dc172763aee039c917fdba03f1e27095f5a7a9)