fprobe: Add nr_maxactive to specify rethook_node pool size

fprobe: Add nr_maxactive to specify rethook_node pool size

[ Upstream commit 59a7a298565aa0ce44ce8e4fbcbb89a19730013a ]

Add nr_maxactive to specify rethook_node pool size. This means
the maximum number of actively running target functions concurrently
for probing by exit_handler. Note that if the running function is
preempted or sleep, it is still counted as 'active'.

Link: https://lkml.kernel.org/r/167526697917.433354.17779774988245113106.stgit@mhiramat.roam.corp.google.com
Cc: Florent Revest <revest@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Stable-dep-of: 700b2b439766 ("fprobe: Fix to ensure the number of active retprobes is not zero")
Signed-off-by: Sasha Levin <sashal@kernel.org>