linux/common-config: set NFS_FS=m
This is built in by default on aarch64, where the defconfig has a bad
habit of including way too much stuff. It's particularly bad in this
case, because it prevents NFS_FSCACHE being set, because FSCACHE is a
module — for NFS_FSCACHE, both NFS and FSCACHE have to be built in, or
both have to be modules.