nfsd: fix error values returned by nfsd4_lockt() when nfsd_open() fails

nfsd_open() already returns an NFS error value; only vfs_test_lock()
result needs to be fed through nfserrno().  Broken by commit 55ef12
(nfsd: Ensure nfsv4 calls the underlying filesystem on LOCKT)
three years ago...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>