dm: take care to copy the space map roots before locking the superblock

dm: take care to copy the space map roots before locking the superblock

commit 5a32083d03fb543f63489b2946c4948398579ba0 upstream.

In theory copying the space map root can fail, but in practice it never
does because we're careful to check what size buffer is needed.

But make certain we're able to copy the space map roots before
locking the superblock.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>