Bluetooth: mgmt: Fix race condition in mesh handling

This patch addresses race conditions in mesh handling within mgmt_util.c.

The functions mgmt_mesh_add and mgmt_mesh_find modify or traverse the
mesh_pending list without locking. This patch uses guard(mutex) with
the existing mgmt_pending_lock to protect the critical sections, as
suggested by maintainers in previous reviews.

Note: The heap buffer overflow fix previously included in earlier
versions of this patch series has already been merged upstream.

Fixes: b338d91703fa ("Bluetooth: Implement support for Mesh")
Cc: stable@vger.kernel.org
Signed-off-by: Maiquel Paiva <maiquelpaiva@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>