gpiosim: fix data race that corrupts heap

gpiosim: fix data race that corrupts heap

Hit this while seeing some heap corruptions when running cargo test on
the Rust bindings.

Took a bit to track down since I first used address sanitizers, but with
the thread sanitizer it becomes obvious immediately (output simplified):

    ==================
    WARNING: ThreadSanitizer: data race (pid=288119)
      Write of size 8 at 0x0000018f1e78 by thread T6:
        #0 id_free /libgpiod/tests/gpiosim/gpiosim.c:141:17
        #1 dev_release /libgpiod/tests/gpiosim/gpiosim.c:600:2
        #2 refcount_dec /libgpiod/tests/gpiosim/gpiosim.c:176:3
        #3 gpiosim_dev_unref /libgpiod/tests/gpiosim/gpiosim.c:671:2
        #4 bank_release /libgpiod/tests/gpiosim/gpiosim.c:873:2
        #5 refcount_dec /libgpiod/tests/gpiosim/gpiosim.c:176:3
        #6 gpiosim_bank_unref /libgpiod/tests/gpiosim/gpiosim.c:941:2
        [...]

      Previous write of size 8 at 0x0000018f1e78 by thread T1:
        #0 id_free /libgpiod/tests/gpiosim/gpiosim.c:141:17
        #1 bank_release /libgpiod/tests/gpiosim/gpiosim.c:878:2
        #2 refcount_dec /libgpiod/tests/gpiosim/gpiosim.c:176:3
        #3 gpiosim_bank_unref /libgpiod/tests/gpiosim/gpiosim.c:941:2
        [...]

      Location is global 'id_del_ctx' of size 16 at 0x0000018f1e70

      Thread T6 'chip::verify::f' (tid=288126, running) created by main thread at:
        #7 test::run_tests::hd53a07a011bd771f /.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:407:21
        [...]

      Thread T1 'chip::open::gpi' (tid=288121, finished) created by main thread at:
        #7 test::run_tests::hd53a07a011bd771f /.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/lib.rs:407:21
        [...]

    SUMMARY: ThreadSanitizer: data race /libgpiod/tests/gpiosim/gpiosim.c:141:17 in id_free
    ==================

This eventually can either lead to leaks or double free's that corrupt
the heap and lead to crashes.

The issue got introduced when a previously local variable that did not
require protection was turned into a global variable.

Fixes: 5e111df2fca5 ("gpiosim: use twalk() instead of twalk_r()")
Signed-off-by: Erik Schilling <erik.schilling@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>