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.