spufs: fix bogosity in S_ISGID handling

clearing everything *except* S_ISGID (including the S_IFDIR, among
other things) is wrong.  Just use init_inode_owner() and be done
with that...

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>