[SPARC64]: Fix most sparse warnings in arch/sparc64/kernel/sys_sparc.c

Sparse still doesn't like the funny cast we make from a scalar to a
"union semun" (which is correct by the C language and in particular
works with the sparc64 calling conventions, but sparse doesn't grok
that yet).

Signed-off-by: David S. Miller <davem@davemloft.net>