pstore/ram: fix resource leak when ioremap() fails

In persistent_ram_iomap(), ioremap() or ioremap_wc() may return NULL on
failure. Currently, if this happens, the function returns NULL without
releasing the memory region acquired by request_mem_region().

This leads to a resource leak where the memory region remains reserved
but unusable.

Additionally, the caller persistent_ram_buffer_map() handles NULL
correctly by returning -ENOMEM, but without this check, a NULL return
combined with request_mem_region() succeeding leaves resources in an
inconsistent state.

This is the ioremap() counterpart to commit 05363abc7625 ("pstore:
ram_core: fix incorrect success return when vmap() fails") which fixed
a similar issue in the vmap() path.

Fixes: 404a6043385d ("staging: android: persistent_ram: handle reserving and mapping memory")
Signed-off-by: Cole Leavitt <cole@unwrap.rs>
Link: https://patch.msgid.link/20260225235406.11790-1-cole@unwrap.rs
Signed-off-by: Kees Cook <kees@kernel.org>