vfs: consult freevnodes in vnlru_kick_cond

If the count is high enough there is no point trying to produce more.
Not going there reduces traffic on the vnode_list mtx.

This further shaves total real time in a test mentioned in:
74be676d87745eb7 ("vfs: drop one vnode list lock trip during vnlru free
recycle") -- 20 instances of find each creating 1 million vnodes, while
total limit is set to 400k.

Time goes down from ~41 to ~35 seconds.

Sponsored by: Rubicon Communications, LLC ("Netgate")