[PATCH v4 4/5] mountmgr: Add support for volume information queries.

[Erich E. Hoover]

This patch allows the mountmgr to handle the volume information
request IRP, allowing volume information to be queried from the
mountmgr.

v4: Reordered (no content change)
v3: Use SystemBuffer instead of UserBuffer (see
8e98dcd42e13bfeb5a2397ff9bf1c7f63c224e23)
v2: Per Zeb's request, now storing the volume with the disk_device so
that the lookup doesn't require a search.

Best,
Erich