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

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>