Pull up following revision(s) (requested by rmind in ticket #863):
sys/sys/vnode.h: revision 1.207
sys/kern/vfs_subr.c: revision 1.379
put a flag bit into v_usecount to prevent vtryget during getcleanvnode.
this fixes the following deadlock.
        a thread doing getcleanvnode:
        pick a vnode
        acqure v_interlock
        v_usecount++
        call vclean
                now, another thread doing cache_lookup:
                picks the vnode
                vtryget succeed
                vn_lock succeed
        now in vclean:
        set VI_XLOCK (too late to be noticed by the competing thread)
        wait on the vnode lock (this might violate locking order)
the use of a flag bit was suggested by Andrew Doran.  PR/41374.

(snj)