btrfs-progs: fix unaligned accesses v2

btrfs-progs: fix unaligned accesses v2

gcc optimizes out the memcpy calls at -O2 and -Os.

Replacing memcpy with memmove does't work - gcc treats memmove
the same way it treats memcpy.

This patch brings in {get|put}_unaligned_le{16|32|64} (using the
packed struct method), and uses them in the failing get/set calls.

On architectures where unaligned accesses are cheap, these unaligned
macros should be optimized out by the compiler.

Signed-off-by: Ben Peddell <klightspeed@killerwolves.net>