firmware: arm_ffa: Fix a possible ffa_linux_errmap buffer overflow

The ffa_linux_errmap buffer access index is supposed to range from 0-8
but it ranges from 1-9 instead. It reads one element out of bounds. It
also changes the success into -EINVAL though ffa_to_linux_errno is never
used in case of success, it is expected to work for success case too.

It is slightly confusing code as the negative of the error code
is used as index to the buffer. Fix it by negating it at the start and
make it more readable.

Link: https://lore.kernel.org/r/20210707134739.1869481-1-sudeep.holla@arm.comReported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_ffa/driver.c

@@ -149,8 +149,10 @@ static const int ffa_linux_errmap[] = {
 
 static inline int ffa_to_linux_errno(int errno)
 {
-	if (errno < FFA_RET_SUCCESS && errno >= -ARRAY_SIZE(ffa_linux_errmap))
-		return ffa_linux_errmap[-errno];
+	int err_idx = -errno;
+
+	if (err_idx >= 0 && err_idx < ARRAY_SIZE(ffa_linux_errmap))
+		return ffa_linux_errmap[err_idx];
 	return -EINVAL;
 }