fscache: Add more tracepoints

Add more tracepoints to fscache, including:

 (*) fscache_page - Tracks netfs pages known to fscache.

 (*) fscache_check_page - Tracks the netfs querying whether a page is
     pending storage.

 (*) fscache_wake_cookie - Tracks cookies being woken up after a page
     completes/aborts storage in the cache.

 (*) fscache_op - Tracks operations being initialised.

 (*) fscache_wrote_page - Tracks return of the backend write_page op.

 (*) fscache_gang_lookup - Tracks lookup of pages to be stored in the write
     operation.

Signed-off-by: David Howells <dhowells@redhat.com>