nfsd4: fix file leak on open_downgrade

Stateid's hold a read reference for a read open, a write reference for a
write open, and an additional one of each for each read+write open.  The
latter wasn't getting put on a downgrade, so something like:

	open RW
	open R
	downgrade to R

was resulting in a file leak.

Also fix an imbalance in an error path.

Regression from 7d947842 "nfsd4: fix
downgrade/lock logic".

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