powerpc/spufs: Fix the type of ret in spufs_arch_write_note

Both the ->dump method and snprintf return an int.  So switch to an
int and properly handle errors from ->dump.

Fixes: 5456ffde ("powerpc/spufs: simplify spufs core dumping")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200610085554.5647-1-hch@lst.de