Since __ATTR_PREALLOC was introduced in v3.19-rc1~78^2~18
it can now be used by md.

This ensure that writing to these sysfs attributes will never
block due to a memory allocation.
Such blocking could become a deadlock if mdmon is trying to
reconfigure an array after a failure prior to re-enabling writes.
Signed-off-by: NeilBrown <neilb@suse.de>