staging: android: persistent_ram: Introduce persistent_ram_vmap()

Factor out vmap logic out of persistent_ram_buffer_map(), this will
make the code a bit more understandable when we'll add support for
non-bootmem memory.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>