book3s64/radix : Handle error conditions properly in radix_vmemmap_populate

Error conditions are not handled properly if altmap is not present
and PMD_SIZE vmemmap_alloc_block_buf fails.

In this patch, if vmemmap_alloc_block_buf fails in the non-altmap
case, we will fall back to the base mapping.

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/7f95fe91c827a2fb76367a58dbea724e811fb152.1750593372.git.donettom@linux.ibm.com