pmem: return EIO on read_pmem() failure

The read_pmem() function uses memcpy_mcsafe() on x86 where an EFAULT
error code indicates a failed read.  Block I/O should use EIO to
indicate failure.  Other pmem code paths (like bad blocks) already use
EIO so let's be consistent.

This fixes compatibility with consumers like btrfs that try to parse the
specific error code rather than treat all errors the same.

Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>