uprobes: protected uprobe lifetime with SRCU

To avoid unnecessarily taking a (brief) refcount on uprobe during
breakpoint handling in handle_swbp for entry uprobes, make find_uprobe()
not take refcount, but protect the lifetime of a uprobe instance with
RCU. This improves scalability, as refcount gets quite expensive due to
cache line bouncing between multiple CPUs.

Specifically, we utilize our own uprobe-specific SRCU instance for this
RCU protection. put_uprobe() will delay actual kfree() using call_srcu().

For now, uretprobe and single-stepping handling will still acquire
refcount as necessary. We'll address these issues in follow up patches
by making them use SRCU with timeout.

Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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