selftests/mm: skip map_populate on weird filesystems

It seems that 9pfs does not allow truncating unlinked files, Mark Brown
has noted that NFS may also behave this way.

It doesn't seem quite right to call this a "bug" but it's probably a
special enough case that it makes sense for the test to just SKIP if it
happens.

Link: https://lkml.kernel.org/r/20250311-mm-selftests-v4-7-dec210a658f5@google.com


Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>