powerpc/procfs: Propagate error of remap_pfn_range()

If the operation fails and userspace is unaware it will access unmapped
memory, crashing the process.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-base-v1-22-b64f0842d512@linutronix.de