Make temporary variable in _rcu_dereference non-const

Make temporary variable in _rcu_dereference non-const

When building the lttng-tools project with Ubuntu's gcc 11, I get the
following error:

      CC       agent.lo
    In file included from /tmp/lttng/include/urcu/arch.h:25,
                     from /tmp/lttng/include/urcu/uatomic.h:23,
                     from /home/simark/src/lttng-tools/src/bin/lttng-sessiond/agent.c:11:
    /home/simark/src/lttng-tools/src/bin/lttng-sessiond/agent.c: In function ‘agent_update’:
    /tmp/lttng/include/urcu/static/pointer.h:96:33: error: argument 2 of ‘__atomic_load’ discards ‘const’ qualifier [-Werror=incompatible-pointer-types]
       96 |                                 __atomic_load(&(p), &_________p1, __ATOMIC_CONSUME);    \
          |                                 ^~~~~~~~~~~~~
    /tmp/lttng/include/urcu/compiler.h:69:70: note: in definition of macro ‘caa_container_of’
       69 |                 const __typeof__(((type *) NULL)->member) * __ptr = (ptr); \
          |                                                                      ^~~
    /tmp/lttng/include/urcu/rculist.h:87:20: note: in expansion of macro ‘cds_list_entry’
       87 |         for (pos = cds_list_entry(rcu_dereference((head)->next), __typeof__(*(pos)), member); \
          |                    ^~~~~~~~~~~~~~
    /tmp/lttng/include/urcu/pointer.h:47:33: note: in expansion of macro ‘_rcu_dereference’
       47 | #define rcu_dereference         _rcu_dereference
          |                                 ^~~~~~~~~~~~~~~~
    /tmp/lttng/include/urcu/rculist.h:87:35: note: in expansion of macro ‘rcu_dereference’
       87 |         for (pos = cds_list_entry(rcu_dereference((head)->next), __typeof__(*(pos)), member); \
          |                                   ^~~~~~~~~~~~~~~
    /home/simark/src/lttng-tools/src/bin/lttng-sessiond/agent.c:1551:9: note: in expansion of macro ‘cds_list_for_each_entry_rcu’
     1551 |         cds_list_for_each_entry_rcu(ctx, &agt->app_ctx_list, list_node) {
          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~

This is because the pointer passed to _rcu_dereference is const (the
pointer itself is const, IIUC, not necessarily the data it points to),
so the temporary _________p1 is also declared as const.  We therefore
can't pass a non-const pointer to it to a function that modifies it.

I applied the trick found here [1] with success to get rid of the
constness of the variable.  With this change, lttng-tools compiles
successfully with gcc 11.

There may be other spots in the headers where this would be needed, but
it is hard to spot them.  I think we would need to write some test file
that pass const pointers to all macros of the API and see if they
compile.

[1] https://stackoverflow.com/a/18067745

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ib43efa7674ecea99cec1c3fffae86829b44a97e5