MFC r314987-r314989, r314993-r314994, r315046-r315049, r315102

r314987:
Use nitems() from sys/param.h.

Reviewed by: asomers
MFC after: 3 weeks.
Differential Revision: https://reviews.freebsd.org/D9944

r314988:
Use nitems() from sys/param.h.

Reviewed by: ume
MFC after: 3 weeks.
Differential Revision: https://reviews.freebsd.org/D9938

r314989:
Use nitems() from sys/param.h and also remove the cast.

Reviewed by: markj
MFC after: 3 weeks.
Differential Revision: https://reviews.freebsd.org/D9937

r314993:
Use nitems() from sys/param.h.

Reviewed by: ngie
MFC after: 3 weeks.
Differential Revision: https://reviews.freebsd.org/D9936

r314994:
Use nitems() from sys/param.h.

Reviewed by: ngie
MFC after: 3 weeks.
Differential Revision: https://reviews.freebsd.org/D9940

r315046:
Use nitems() from sys/param.h

Reviewed by: jhb
MFC after: 3 weeks.
Differential Revision: https://reviews.freebsd.org/D9941

r315047:
Use nitems() from sys/param.h.

Reviewed by: np
MFC after: 3 weeks.
Differential Revision: https://reviews.freebsd.org/D9945

r315048:
Use nitems() from sys/param.h.

MFC after: 3 weeks.

r315049:
Bring back the cast removed in my previous commit to allow us build vmstat
with WARNS 2. This cast was first introduced at r87690.

Reported by: bde, pfg and ngie
MFC after: 3 weeks.

r315102:
Use nitems() from sys/param.h.

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D9943