codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations

If slabobj_ext vector allocation for a slab object fails and later on it
succeeds for another object in the same slab, the slabobj_ext for the
original object will be NULL and will be flagged in case when
CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled.
Mark failed slabobj_ext vector allocations using a new objext_flags flag
stored in the lower bits of slab->obj_exts. When new allocation succeeds
it marks all tag references in the same slabobj_ext vector as empty to
avoid warnings implemented by CONFIG_MEM_ALLOC_PROFILING_DEBUG checks.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>