nfsd: avoid races with wake_up_var()

wake_up_var() needs a barrier after the important change is made in the
var and before wake_up_var() is called, else it is possible that a wake
up won't be sent when it should.

In each case here the var is changed in an "atomic" manner, so
smb_mb__after_atomic() is sufficient.

In one case the important change (removing the lease) is performed
*after* the wake_up, which is backwards.  The code survives in part
because the wait_var_event is given a timeout.

This patch adds the required barriers and calls destroy_delegation()
*before* waking any threads waiting for the delegation to be destroyed.

Signed-off-by: NeilBrown <[email protected]>
Reviewed-by: Jeff Layton <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>

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