[POWERPC] spufs: fix possible memory corruption is spufs_mem_write

Due to a buggy unsigned comparison, it was possible to write
beyond the end of the local store file in spufs under some
circumstances.

This rewrites the buggy function to look more like
simple_copy_from_buffer.

Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Cc: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>