mdadm: Block SIGCHLD processes before starting children

mdadm: Block SIGCHLD processes before starting children

There is a small race condition noticed during code review, but
never actully hit in practice, with the write_zero feature.

If a write zeros fork finishes quickly before wait_for_zero_forks()
gets called, then the SIGCHLD will be delivered before the signalfd
is setup.

While this is only theoretical, fix this by blocking the SIGCHLD
signal before forking any children.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>