FROMGIT: pinmux: fix race causing mux_owner NULL with active mux_usecount

commit 5a3e85c3 ("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.

Bug: 430525600
Bug: 433603976
Fixes: 5a3e85c3 ("pinmux: Use sequential access to access desc->pinmux data")
Link: https://lore.kernel.org/lkml/20250708-pinmux-race-fix-v2-1-8ae9e8a0d1a1@oss.qualcomm.com/


Signed-off-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>
(cherry picked from commit 0b075c01
 git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-pinctrl.git

 for-next)
Change-Id: Iec29ea201ef0fc3d205bbc4f1a90cb5a56a62039
Signed-off-by: Mukesh Ojha <mukesh.ojha@oss.qualcomm.com>
(cherry picked from commit 6b4fffd5)
(cherry picked from commit e7f2a983)