mount_nullfs: rename a local variable

The "source" variable was introduced in r26072, probably as the
traditional counterpart to "target". But the "source"/"target" names
suggest the opposite of their actual meaning. With ln, for example, the
source is the real file and the target is the newly created link. In
mount_nullfs the meaning is the opposite: the target is the existing
file system and the source is the newly created mountpoint. Better to
use "target"/"mountpoint" terminology, which matches the man page.

MFC after: 6 weeks
Sponsored by: Axcient