missing `/sys/kernel/address_bits` causes failing Linux tests

[jtracey]

Three tests added in #6220 use the /sys/kernel/address_bits file on Linux (test_cp_default_virtual_file(), test_cp_debug_sparse_always_sparse_virtual_file(), and test_cp_debug_default_sparse_virtual_file()). This file was only added a year ago, so those tests are failing on any distro with a kernel older than that. E.g., I don't have that file on Debian 12 (bookworm). @AnirbanHalder654322, is there anything special about this file (other than being virtual) for those tests, and if so, are there any other older files that could be used instead?

[BenWiederhake]

I believe /sys/kernel/address_bits was chosen according to these criteria:

1. It is a virtual file, i.e. has zero disk usage, occupies zero blocks, and also is "empty" in various stats
2. It is not empty when read
3. It is present on basically every platform

Whelp, turns out that criterion 3 was not met. I guess we should replace the file by basically any other file in /sys/, and this time spend more effort to pick something that has already existed for many many Linux kernel versions.

Edit: Phrasing

[AnirbanHalder654322]

The file itself is not special. The criteria is exactly as @BenWiederhake mentioned and can be replaced by any file fulfilling those criteria.

Edit: The file would also need to be "acting" as a sparse-file i.e. the size of the file should be greater than the minimum block_size supported by GNU i.e. 512 to pass those specific tests.

[AnirbanHalder654322]

Any easy solution could be to just use the file that the gnu test-suite uses for testing in tests/head/head-c.sh i.e. /sys/kernel/profiling , this fulfills our criteria and should be present in most older platforms as well.

[jtracey]

Fixed in #6294.