nfsrahead SIGBUS crash on aarch64 for non-NFS block devices

nfsrahead crashes with SIGBUS on aarch64 when invoked by udev for
non-NFS block devices (virtio disks, loop devices, etc).

The udev rule 99-nfsrahead.rules fires for all block devices with no
filter for NFS-backed devices. nfsrahead is supposed to exit gracefully
for non-NFS devices but instead crashes in __libc_free() due to a
misaligned pointer — a hard fault on aarch64, silent on x86_64.

Stack trace:
#0  __libc_free (libc.so.6)
#1  nfsrahead + 0x2074
#2  __libc_start_main (libc.so.6)
#3  nfsrahead + 0x22f0

Triggered for: 254:0 (virtio disk), 7:1, 7:2 (loop devices)

Fix needed:
1. nfsrahead should detect non-NFS devices and exit 0 gracefully
2. Fix the misaligned pointer in the cleanup/exit path on aarch64