NFS: fix handling of malloc failure during nfs_flush_multi()

Cleanup of the allocated list entries should not call
put_nfs_open_context() on each entry, as the context will
always be NULL, causing an oops.

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>