fscache not properly guarded

[ericvh]

there is lots of uses of fscache code that aren't properly guarded by ifdefs NOR guarded by mount options. Need a full code review to find and fix these.

[ericvh]

Validation & remediation plan

Type: code hygiene / build & runtime guarding of fscache. Overlaps #27.

Still a problem?

1. Full audit of fs/9p for fscache_*, linux/fscache.h includes, and cookie fields outside #ifdef CONFIG_9P_FSCACHE (headers already use netfs cookie wrappers — verify .c files).
2. Build with CONFIG_9P_FSCACHE=n and =y (allyesconfig snippet); fix link/include failures.
3. Runtime half: same as fscache operations not guarded by cache=fscache #27 — operations must be mount-option gated, not only Kconfig gated.

Address plan

1. Single patch series: Kconfig ifdef cleanup + CACHE_FSCACHE runtime guards + helper inlines in cache.h.
2. Drop redundant includes (vfs_dir.c / vfs_super.c) where netfs wrappers suffice.
3. CI: build both FSCACHE variants in kernel publish workflow.
4. Close fscache operations not guarded by cache=fscache #27 and fscache not properly guarded #18 together when audit checklist is empty.