nfsd4: fix break_lease flags on nfsd open

Thanks to Casey Bodley for pointing out that on a read open we pass 0,
instead of O_RDONLY, to break_lease, with the result that a read open is
treated like a write open for the purposes of lease breaking!

Reported-by: Casey Bodley <cbodley@citi.umich.edu>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>