hfsplus: fix compiler warning on PowerPC

Commit a99b7069 ("hfsplus: Fix undefined __divdi3 in
hfsplus_init_header_node()") introduced do_div() to xattr.c and the
warning below too.

As Geert remarked: "tmp" is "loff_t" which is "__kernel_loff_t", which
is "long long", i.e.  signed, while include/asm-generic/div64.h compares
its type with "uint64_t".  As inode sizes are positive, it should be
safe to change the type of "tmp" to "u64".

  In file included from
  arch/powerpc/include/asm/div64.h:1:0,
                    from include/linux/kernel.h:124,
                    from include/asm-generic/bug.h:13,
                    from arch/powerpc/include/asm/bug.h:127,
                    from include/linux/bug.h:4,
                    from include/linux/thread_info.h:11,
                    from include/asm-generic/preempt.h:4,
                    from arch/powerpc/include/generated/asm/preempt.h:1,
                    from include/linux/preempt.h:18,
                    from include/linux/spinlock.h:50,
 ...