Always assume SKIP_GONE_DEVS behaviour and kill the flag
...i.e. GET_DEVS == (GET_DEVS|SKIP_GONE_DEVS)
A null pointer dereference in Incremental.c can be triggered by
replugging a disk while the old name is in use. When mdadm -I is called
on the new disk we fail the call to sysfs_read(). I audited all the
locations that use GET_DEVS and it appears they can tolerate missing a
drive. So just make SKIP_GONE_DEVS the default behaviour.
Also fix up remaining unchecked usages of the sysfs_read() return value.
Reported-by: Dave Jiang <dave.jiang@intel.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com>