Tweaks for r284178:

Do not include machine/atomic.h explicitely, the header is already included
by sys/systm.h.

Force inlining of tc_getgen() and tc_setgen(). The functions are used
more than once, which causes compilers with non-aggressive inlining
policies to generate calls.

Suggested by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 1 week