nfsd4: remember to put RW access on stateid destruction

commit 499f3edc upstream.

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".

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>