lockd: fix nlm_close_files

This loop condition tries a bit too hard to be clever. Just test for
the two indices we care about explicitly.

Cc: J. Bruce Fields <bfields@fieldses.org>
Fixes: 7f024fcd ("Keep read and write fds with each nlm_file")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>