fscache operations not guarded by cache=fscache

[ericvh]

there are lots of fscache operations that will be called if fscache is configured even if it isn't enabled at mount time.

[ericvh]

Validation & remediation plan

Type: correctness / overhead when CONFIG_9P_FSCACHE=y but mount is not cache=fscache. Closely related to #18.

Still a problem?

1. Build with CONFIG_9P_FSCACHE=y. Mount cache=none and cache=mmap.
2. Code: v9fs_cache_inode_get_cookie() is invoked unconditionally from inode init; session volume is only created when CACHE_FSCACHE is set — confirm whether fscache_acquire_cookie(NULL, …) is a no-op or still does work / warns.
3. Audit all fscache_* call sites in fs/9p for missing v9ses->cache & CACHE_FSCACHE guards (use_cookie, unuse_cookie, resize_cookie, relinquish).
4. Runtime: perf/ftrace around inode create and close; watch for fscache cachefiles activity with non-fscache mounts.

Address plan

1. Early-return in v9fs_cache_inode_get_cookie() unless CACHE_FSCACHE and session volume non-NULL.
2. Unify guards with helpers: v9fs_fscache_enabled(v9ses) used at every callsite.
3. Coccinelle/semantic patch + v9fs/test build matrix: FSCACHE=y/n × cache modes; assert no cachefiles I/O unless fscache mount.
4. Consider merging resolution with fscache not properly guarded #18 into one cleanup series.