print_binder_proc() drops proc->inner_lock and calls
binder_alloc_print_allocated() which acquires proc->alloc->mutex.
Likewise, print_binder_stats() calls print_binder_proc_stats() which
drops its locks to call binder_alloc_print_pages() which also acquires
proc->alloc->mutex. So binder_procs_lock needs to be a mutex since it
can block on proc->alloc->mutex.