fscache: Fix invalidation/lookup race

In the Linux kernel, the following vulnerability has been resolved:

fscache: Fix invalidation/lookup race

If an NFS file is opened for writing and closed, fscache_invalidate() will

be asked to invalidate the file - however, if the cookie is in the

LOOKING_UP state (or the CREATING state), then request to invalidate

doesn't get recorded for fscache_cookie_state_machine() to do something

with.

Fix this by making __fscache_invalidate() set a flag if it sees the cookie

is in the LOOKING_UP state to indicate that we need to go to invalidation.

Note that this requires a count on the n_accesses counter for the state

machine, which that will release when it's done.

fscache_cookie_state_machine() then shifts to the INVALIDATING state if it

sees the flag.

Without this, an nfs file can get corrupted if it gets modified locally and

then read locally as the cache contents may not get updated.