fscache: Implement volume-level access helpers

Add a pair of helper functions to manage access to a volume, pinning the
volume in place for the duration to prevent cache withdrawal from removing
it:

	bool fscache_begin_volume_access(struct fscache_volume *volume,
					 enum fscache_access_trace why);
	void fscache_end_volume_access(struct fscache_volume *volume,
				       enum fscache_access_trace why);

The way the access gate on the volume works/will work is:

  (1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
      then we return false to indicate access was not permitted.

  (2) If the cache tests as live, then we increment the volume's n_accesses
      count and then recheck the cache liveness, ending the access if it
      ceased to be live.

  (3) When we end the access, we decrement the volume's n_accesses and wake
      up the any waiters if it reaches 0.

  (4) Whilst the cache is caching, the volume's n_accesses is kept
      artificially incremented to prevent wakeups from happening.

  (5) When the cache is taken offline, the state is changed to prevent new
      accesses, the volume's n_accesses is decremented and we wait for it to
      become 0.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819594158.215744.8285859817391683254.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906894315.143852.5454793807544710479.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967095028.1823006.9173132503876627466.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021501546.640689.9631510472149608443.stgit@warthog.procyon.org.uk/ # v4