md: lock access to rdev attributes properly

When we access attributes of an rdev (component device on an md array) through
sysfs, we really need to lock the array against concurrent changes.  We
currently do that when we change an attribute, but not when we read an
attribute.  We need to lock when reading as well else rdev->mddev could become
NULL while we are accessing it.

So add appropriate locking (mddev_lock) to rdev_attr_show.

rdev_size_store requires some extra care as well as it needs to unlock the
mddev while scanning other mddevs for overlapping regions.  We currently
assume that rdev->mddev will still be unchanged after the scan, but that
cannot be certain.  So take a copy of rdev->mddev for use at the end of the
function.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>