uprobes: Introduce uprobe_apply()

Currently it is not possible to change the filtering constraints after
uprobe_register(), so a consumer can not, say, start to trace a task/mm
which was previously filtered out, or remove the no longer needed bp's.

Introduce uprobe_apply() which simply does register_for_each_vma() again
to consult uprobe_consumer->filter() and install/remove the breakpoints.
The only complication is that register_for_each_vma() can no longer
assume that uprobe->consumers should be consulter if is_register == T,
so we change it to accept "struct uprobe_consumer *new" instead.

Unlike uprobe_register(), uprobe_apply(true) doesn't do "unregister" if
register_for_each_vma() fails, it is up to caller to handle the error.

Note: we probably need to cleanup the current interface, it is strange
that uprobe_apply/unregister need inode/offset. We should either change
uprobe_register() to return "struct uprobe *", or add a private ->uprobe
member in uprobe_consumer. And in the long term uprobe_apply() should
take a single argument, uprobe or consumer, even "bool add" should go
away.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

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