uprobes: Rationalize the usage of filter_chain()

filter_chain() was added into install_breakpoint/remove_breakpoint to
simplify the initial changes but this is sub-optimal.

This patch shifts the callsite to the callers, register_for_each_vma()
and uprobe_mmap(). This way:

- It will be easier to add the new arguments. This is the main reason,
  we can do more optimizations later.

- register_for_each_vma(is_register => true) can be optimized, we only
  need to consult the new consumer. The previous consumers were already
  asked when they called uprobe_register().

This patch also moves the MMF_HAS_UPROBES check from remove_breakpoint(),
this allows to avoid the potentionally costly filter_chain(). Note that
register_for_each_vma(is_register => false) doesn't really need to take
->consumer_rwsem, but I don't think it makes sense to optimize this and
introduce filter_chain_lockless().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

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