powerpc/signal32: Fix erroneous SIGSEGV on RT signal return

Return of user_read_access_begin() is tested the wrong way,
leading to a SIGSEGV when the user address is valid and likely
an Oops when the user address is bad.

Fix the test.

Fixes: 887f3ceb ("powerpc/signal32: Convert do_setcontext[_tm]() to user access block")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a29aadc54c93bcbf069a83615fa102ca0f59c3ae.1619185912.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/signal_32.c

@@ -971,7 +971,7 @@ static int do_setcontext(struct ucontext __user *ucp, struct pt_regs *regs, int
 	sigset_t set;
 	struct mcontext __user *mcp;
 
-	if (user_read_access_begin(ucp, sizeof(*ucp)))
+	if (!user_read_access_begin(ucp, sizeof(*ucp)))
 		return -EFAULT;
 
 	unsafe_get_sigset_t(&set, &ucp->uc_sigmask, failed);
@@ -1009,7 +1009,7 @@ static int do_setcontext_tm(struct ucontext __user *ucp,
 	u32 cmcp;
 	u32 tm_cmcp;
 
-	if (user_read_access_begin(ucp, sizeof(*ucp)))
+	if (!user_read_access_begin(ucp, sizeof(*ucp)))
 		return -EFAULT;
 
 	unsafe_get_sigset_t(&set, &ucp->uc_sigmask, failed);