ceph: Do not do invalidate if the filesystem is mounted nofsc

Previously we would always try to enqueue work even if the filesystem is not
mounted with fscache enabled (or the file has no cookie). In the case of the
filesystem mouned nofsc (but with fscache compiled in) this would lead to a
crash.

Signed-off-by: Milosz Tanski <milosz@adfin.com>

fs/ceph/cache.c

@@ -374,8 +374,12 @@ static void ceph_revalidate_work(struct work_struct *work)
 
 void ceph_queue_revalidate(struct inode *inode)
 {
+	struct ceph_fs_client *fsc = ceph_sb_to_client(inode->i_sb);
 	struct ceph_inode_info *ci = ceph_inode(inode);
 
+	if (fsc->revalidate_wq == NULL || ci->fscache == NULL)
+		return;
+
 	ihold(inode);
 
 	if (queue_work(ceph_sb_to_client(inode->i_sb)->revalidate_wq,