pinctrl: Duplicate user memory in one go in pinmux_select()

Current code is suboptimal in three ways:
1) it explicitly terminates the string which is not needed;
2) it might provoke additional faults, because asked lenght might be
   bigger than the real one;
3) it consumes more than needed lines in the source.

Instead of using kmalloc() + strncpy_from_user() + terminating, just
utilize memdup_user_nul().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230604131215.78847-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Author: andy-shev

drivers/pinctrl/pinmux.c

@@ -692,14 +692,9 @@ static ssize_t pinmux_select(struct file *file, const char __user *user_buf,
 	if (len > PINMUX_SELECT_MAX)
 		return -ENOMEM;
 
-	buf = kzalloc(PINMUX_SELECT_MAX, GFP_KERNEL);
-	if (!buf)
-		return -ENOMEM;
-
-	ret = strncpy_from_user(buf, user_buf, PINMUX_SELECT_MAX);
-	if (ret < 0)
-		goto exit_free_buf;
-	buf[len-1] = '\0';
+	buf = memdup_user_nul(user_buf, len);
+	if (IS_ERR(buf))
+		return PTR_ERR(buf);
 
 	/* remove leading and trailing spaces of input buffer */
 	gname = strstrip(buf);