vfs: don't provoke recycling non-free vnodes without a good reason

If the total number of free vnodes is at or above target, there is no
point creating more of them.

Tested by: pho (in a bigger patch)

(cherry picked from commit 8733bc277a383cf59f38a83956f4f523869cfc90)
(cherry picked from commit 71a06765dc67090735293027136c8a0b5fc8c173)

Approved by: re (gjb)