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

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.

Signed-off-by: Ruipeng Qi <ruipengqi3@gmail.com>
Link: https://patch.msgid.link/20260203020358.3315299-1-ruipengqi3@gmail.com
Signed-off-by: Kees Cook <kees@kernel.org>