nfsd4: remember to put RW access on stateid destruction

Without this, for example,

	open read
	open read+write
	close

will result in a struct file leak.

Regression from 7d947842

 "nfsd4: fix
downgrade/lock logic".

Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>