Avoid the GEOM topology lock recursion when we automatically expand a pool.
The steps to reproduce the problem:
mdconfig -a -t swap -s 3g -u 0
gpart create -s GPT md0
gpart add -t freebsd-zfs -s 1g md0
zpool create -o autoexpand=on foo md0p1
gpart resize -i 1 -s 2g md0