MFC r324610:
Reduce traffic on vm_cnt.v_free_count The variable is modified with the highly contended page free queue lock. It unnecessarily shares a cacheline with purely read-only fields and is re-read after the lock is dropped in the page allocation code making the hold time longer. Pad the variable just like the others and store the value as found with the lock held instead of re-reading. Provides a modest 1%-ish speed up in concurrent page faults.
Due to KBI constraints the field is not moved in this commit, only re-read is
avoided.