time(3): Optimize tvtohz() function.

time(3): Optimize tvtohz() function.

List of changes:
- Use integer multiplication instead of long multiplication, because the result is an integer.
- Remove multiple if-statements and predict new if-statements.
- Rename local variable name, "ticks" into "retval" to avoid shadowing
the system "ticks" global variable.

Reviewed by:	kib@ and imp@
MFC after:	1 week
Sponsored by:	NVIDIA Networking
Differential Revision:  https://reviews.freebsd.org/D36859