powerpc/math: Fix missing __user qualifier for get_user() and other sparse warnings

Sparse reports the following problems:

arch/powerpc/math-emu/math.c:228:21: warning: Using plain integer as NULL pointer
arch/powerpc/math-emu/math.c:228:31: warning: Using plain integer as NULL pointer
arch/powerpc/math-emu/math.c:228:41: warning: Using plain integer as NULL pointer
arch/powerpc/math-emu/math.c:228:51: warning: Using plain integer as NULL pointer
arch/powerpc/math-emu/math.c:237:13: warning: incorrect type in initializer (different address spaces)
arch/powerpc/math-emu/math.c:237:13:    expected unsigned int [noderef] __user *_gu_addr
arch/powerpc/math-emu/math.c:237:13:    got unsigned int [usertype] *
arch/powerpc/math-emu/math.c:226:1: warning: symbol 'do_mathemu' was not declared. Should it be static?

Add missing __user qualifier when casting pointer used in get_user()

Use NULL instead of 0 to initialise opX local variables.

Add a prototype for do_mathemu() (Added in processor.h like sparc)
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e4d1aae7604d89c98a52dfd8ce8443462e595670.1615809591.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/processor.h

@@ -413,6 +413,8 @@ extern int fix_alignment(struct pt_regs *);
 #define NET_IP_ALIGN	0
 #endif
 
+int do_mathemu(struct pt_regs *regs);
+
 #endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_POWERPC_PROCESSOR_H */

arch/powerpc/kernel/traps.c

@@ -1405,7 +1405,6 @@ int is_valid_bugaddr(unsigned long addr)
 static int emulate_math(struct pt_regs *regs)
 {
 	int ret;
-	extern int do_mathemu(struct pt_regs *regs);
 
 	ret = do_mathemu(regs);
 	if (ret >= 0)

arch/powerpc/math-emu/math.c

@@ -225,7 +225,7 @@ record_exception(struct pt_regs *regs, int eflag)
 int
 do_mathemu(struct pt_regs *regs)
 {
-	void *op0 = 0, *op1 = 0, *op2 = 0, *op3 = 0;
+	void *op0 = NULL, *op1 = NULL, *op2 = NULL, *op3 = NULL;
 	unsigned long pc = regs->nip;
 	signed short sdisp;
 	u32 insn = 0;
@@ -234,7 +234,7 @@ do_mathemu(struct pt_regs *regs)
 	int type = 0;
 	int eflag, trap;
 
-	if (get_user(insn, (u32 *)pc))
+	if (get_user(insn, (u32 __user *)pc))
 		return -EFAULT;
 
 	switch (insn >> 26) {