nfsd: clean up reset_union_bmap_deny

Fix the "deny" argument type, and start the loop at 1. The 0 iteration
is always a noop.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>