[POWERPC] spufs: fix memory leak on coredump

Dynamically allocated read/write buffer in spufs_arch_write_note() will
not be freed. Convert it to get_free_page at the same time.

Cc: Akinobu Mita <mita@fixstars.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>