Drivers: hv: Ballon: Make pressure posting thread sleep interruptibly

The non-interruptible sleep of the memory pressure posting thread
results in higher reported load average. Make this sleep interruptible.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>