Add two new counters for vnode life cycle events:

• vfs.recycles counts the number of vnodes forcefully recycled to avoid exceeding kern.maxvnodes.
• vfs.vnodes_created counts the number of vnodes created by successful calls to getnewvnode().

Differential Revision: https://reviews.freebsd.org/D1671
Reviewed by: kib
MFC after: 1 week