In nfs_upgrade_vnlock(), assert that the vnode is locked. It is for all
pathes, as far as I see and testing seems to confirm it. Comparision of
old_lock with LK_SHARED make sense only if vnode is locked by current
thread.

When downgrading, pass LK_RETRY to the vn_lock(), since otherwise
vn_lock() unlocks the doomed vnode, causing extra unlock.

Reported and tested by: pho
Approved by: re (rwatson)
MFC after: 3 weeks