During the initial label of a virstor device, write out the allocation
map in chunks if it is large (> 1 MB) in order to avoid large mallocs.

Even though the kernel virstor geom may still do a large malloc to
represent the allocation map, this may still be useful to avoid a
ulimit.

Sponsored by: Dell EMC Isilon

I am unsure if geom virstor is generally in use these days. My web
searches have not turned up much. I am interested in using it to
simulate very large drives for test purposes. In my scenario, I may
need a very large map (gigabytes). So I am looking at making virstor
changes to support this.

New test in D45535

% sudo kyua debug -k /usr/tests/sys/Kyuafile geom/class/virstor/virstor_test:basic
Executing command [ gvirstor label -v -s 64M -m 512 virstor.bjyhzA /dev/md0 ]
Executing command [ gvirstor destroy virstor.bjyhzA ]

Destroying test virstor device: virstor.bjyhzA
Removing test memory disk: md0
geom/class/virstor/virstor_test:basic -> passed