pstore: ram_core: fix incorrect success return when vmap() fails

In the Linux kernel, the following vulnerability has been resolved:

pstore: ram_core: fix incorrect success return when vmap() fails

In persistent_ram_vmap(), vmap() may return NULL on failure.

If offset is non-zero, adding offset_in_page(start) causes the function

to return a non-NULL pointer even though the mapping failed.

persistent_ram_buffer_map() therefore incorrectly returns success.

Subsequent access to prz->buffer may dereference an invalid address

and cause crashes.

Add proper NULL checking for vmap() failures.