Wrong usage of BMM cache flushing

Wrong usage of flushing API.
Instead of using the dmac_map_area, dmac_flush_range must be used since the first is restricted to kernel space only and the latter can accept both kernel and user area.
The reason is the BMM allocates and maps buffers into a user area where the addresses that can be used are not kernel.

This fixes an issue that cache flushing BMM buffers allocated to vmeta makes a kernel panic

Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>