Drivers: hv: Create debugfs file with hyper-v balloon usage information

Allow the guest to know how much it is ballooned by the host.
It is useful when debugging out of memory conditions.

When host gets back memory from the guest it is accounted
as used memory in the guest but the guest have no way to know
how much it is actually ballooned.

Expose current state, flags and max possible memory to the guest.
While at it - fix a 10+ years old typo.

Signed-off-by: Alexander Atanasov <alexander.atanasov@virtuozzo.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20220711181825.52318-1-alexander.atanasov@virtuozzo.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>