NFS_FSCACHE and CIFS_FSCACHE are required to enable the local cache support for
NFS and CIFS (the 'fsc' mount option). The reasoning for enabling it from 3.2
and forward is that at least Ubuntu 12.04 (linux 3.2) has it turned on. Could
probably be enabled for earlier kernels too.
the kernel itself (and busybox's cifs mount code) are no longer able
to do this in some/most cases and will error out saying:
"CIFS VFS: connecting to DFS root not implemented yet""
Nixos' qemu-vm target is hurt by this, as it wants to mount /nix/store
via cifs very early in the boot process.
this commit just marks the problematic kernels.
An associated commit in nixos will use this info to fix the problem.
