powerpc/spufs: simplify spufs core dumping

Replace the coredump ->read method with a ->dump method that must call
dump_emit itself.  That way we avoid a buffer allocation an messing with
set_fs() to call into code that is intended to deal with user buffers.
For the ->get case we can now use a small on-stack buffer and avoid
memory allocations as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>