selftests/mm: add commentary about 9pfs bugs

As discussed here:

https://lore.kernel.org/lkml/Z9RRkL1hom48z3Tt@google.com/

This code could benefit from some more commentary.

To avoid needing to comment the same thing in multiple places (I guess
more of these SKIPs will need to be added over time, for now I am only
like 20% of the way through Project Run run_vmtests.sh Successfully), add
a dummy "skip tests for this specific reason" function that basically just
serves as a hook to hang comments on.

Link: https://lkml.kernel.org/r/20250317-9pfs-comments-v1-1-9ac96043e146@google.com


Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>