pinmux: fix race causing mux_owner NULL with active mux_usecount

In the Linux kernel, the following vulnerability has been resolved:

pinmux: fix race causing mux_owner NULL with active mux_usecount

commit 5a3e85c3c397 ("pinmux: Use sequential access to access

desc->pinmux data") tried to address the issue when two client of the

same gpio calls pinctrl_select_state() for the same functionality, was

resulting in NULL pointer issue while accessing desc->mux_owner.

However, issue was not completely fixed due to the way it was handled

and it can still result in the same NULL pointer.

The issue occurs due to the following interleaving:

cpu0 (process A) cpu1 (process B)

pin_request() { pin_free() {

mutex_lock()

desc->mux_usecount--; //becomes 0

..

mutex_unlock()

mutex_lock(desc->mux)

desc->mux_usecount++; // becomes 1

desc->mux_owner = owner;

mutex_unlock(desc->mux)

mutex_lock(desc->mux)

desc->mux_owner = NULL;

mutex_unlock(desc->mux)

This sequence leads to a state where the pin appears to be in use

(`mux_usecount == 1`) but has no owner (`mux_owner == NULL`), which can

cause NULL pointer on next pin_request on the same pin.

Ensure that updates to mux_usecount and mux_owner are performed

atomically under the same lock. Only clear mux_owner when mux_usecount

reaches zero and no new owner has been assigned.