ceph: don't invalidate page cache when inode is no longer used

ceph_check_caps() invalidate page cache when inode is not used
by any open file. This behaviour is not friendly for workload
that repeatly read files.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index fe2c982..a7a967a 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -1430,6 +1430,13 @@
 		if ((used | wanted) & CEPH_CAP_ANY_WR)
 			goto out;
 	}
+	/* The inode has cached pages, but it's no longer used.
+	 * we can safely drop it */
+	if (wanted == 0 && used == CEPH_CAP_FILE_CACHE &&
+	    !(oissued & CEPH_CAP_FILE_CACHE)) {
+	  used = 0;
+	  oissued = 0;
+	}
 	if ((used | wanted) & ~oissued & mine)
 		goto out;   /* we need these caps */
 
@@ -1438,7 +1445,7 @@
 		/* we aren't the only cap.. just remove us */
 		__ceph_remove_cap(cap, true);
 	} else {
-		/* try to drop referring dentries */
+		/* try dropping referring dentries */
 		spin_unlock(&ci->i_ceph_lock);
 		d_prune_aliases(inode);
 		dout("trim_caps_cb %p cap %p  pruned, count now %d\n",