gpio: shared: fix deadlock on shared proxy's parent removal

gpio: shared: fix deadlock on shared proxy's parent removal

Commit 710abda58055 ("gpio: shared: call gpio_chip::of_xlate() if set")
used the mutex embedded in struct gpio_shared_entry to protect the
offset field which now can be modified after assignment. The critical
section however is too wide and introduced a potential deadlock on the
removal of the shared GPIO proxy's parent.

Make the critical section shorter - only protect the offset when it's
being read.

While at it: mention the fact that the entry lock is now also used to
protect against concurrent access to the offset field in the structure's
documentation.

Cc: stable@vger.kernel.org
Fixes: 710abda58055 ("gpio: shared: call gpio_chip::of_xlate() if set")
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260522-gpio-shared-deadlock-v1-1-76bca088f8c0@oss.qualcomm.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>