scripts: get_abi.pl: add a graph to speedup the undefined algorithm

scripts: get_abi.pl: add a graph to speedup the undefined algorithm

Searching for symlinks is an expensive operation with the current
logic, as it is at the order of O(n^3). In practice, running the
check spends 2-3 minutes to check all symbols.

Fix it by storing the directory tree into a graph, and using
a Breadth First Search (BFS) to find the links for each sysfs node.

With such improvement, it can now report issues with ~11 seconds
on my machine.

It comes with a price, though: there are more symbols reported
as undefined after this change. I suspect it is due to some
sysfs circular loops that are dropped by BFS. Despite such
increase, it seems that the reports are now more coherent.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/f5c1e7b14a27132821c08f0459ba9aea3ed69028.1631957565.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>