start_mdmon: provide more dynamic way to close-all-fds

start_mdmon: provide more dynamic way to close-all-fds

When forking mdmon we need to close all other fds because we don't
use O_CLOEXEC yet.
Any approach will be fairly arbitrary, but as we can expect fds to be
fairly dense, closing until we find a set number that don't need
closing is possible safer than only closing the first 100.
So keep closing until we find 20 that are already closed.

Signed-off-by: NeilBrown <neilb@suse.de>