config_opts.h: increase xfs,btrfs wbthrottle defaults

Also increase fd limit defaults to accomodate the larger number
of fds.

Fixes: #5692
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Mark Nelson <mark.nelson@inktank.com>