zfs: Fix unaligned read of uint64

zfs: Fix unaligned read of uint64

Without this patch, when trying to boot zfs using U-Boot on a Jetson TX2
NX (which is aarch64), I get a CPU reset error like so:

"Synchronous Abort" handler, esr 0x96000021
elr: 00000000800c9000 lr : 00000000800c8ffc (reloc)
elr: 00000000fff77000 lr : 00000000fff76ffc
x0 : 00000000ffb40f04 x1 : 0000000000000000
x2 : 000000000000000a x3 : 0000000003100000
x4 : 0000000003100000 x5 : 0000000000000034
x6 : 00000000fff9cc6e x7 : 000000000000000f
x8 : 00000000ff7f84a0 x9 : 0000000000000008
x10: 00000000ffb40f04 x11: 0000000000000006
x12: 000000000001869f x13: 0000000000000001
x14: 00000000ff7f84bc x15: 0000000000000010
x16: 0000000000002080 x17: 00000000001fffff
x18: 00000000ff7fbdd8 x19: 00000000ffb405f8
x20: 00000000ffb40dd0 x21: 00000000fffabe5e
x22: 000000ea77940000 x23: 00000000ffb42090
x24: 0000000000000000 x25: 0000000000000000
x26: 0000000000000000 x27: 0000000000000000
x28: 0000000000bab10c x29: 00000000ff7f85f0

Code: d00001a0 9103a000 94006ac6 f9401ba0 (f9400000)
Resetting CPU ...

This happens when be64_to_cpu() is called on a value that exists at a
memory address that's 4 byte aligned but not 8 byte aligned (e.g. an
address ending in 04). The call stack where that happens is:
check_pool_label() ->
zfs_nvlist_lookup_uint64(vdevnvlist, ZPOOL_CONFIG_ASHIFT,...) ->
be64_to_cpu()

Signed-off-by: Phaedrus Leeds <mwleeds@mailtundra.com>
Fixes: 4d3c95f5ea7c ("zfs: Add ZFS filesystem support")