ceph: avoid calling ceph_renew_caps() infinitely

__ceph_caps_mds_wanted() ignores caps from stale session. So the
return value of __ceph_caps_mds_wanted() can keep the same across
ceph_renew_caps(). This causes try_get_cap_refs() to keep calling
ceph_renew_caps(). The fix is ignore the session valid check for
the try_get_cap_refs() case. If session is stale, just let the
caps requester sleep.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
diff --git a/fs/ceph/file.c b/fs/ceph/file.c
index 045d30d..beb24f8 100644
--- a/fs/ceph/file.c
+++ b/fs/ceph/file.c
@@ -283,7 +283,7 @@ int ceph_open(struct inode *inode, struct file *file)
 	spin_lock(&ci->i_ceph_lock);
 	if (__ceph_is_any_real_caps(ci) &&
 	    (((fmode & CEPH_FILE_MODE_WR) == 0) || ci->i_auth_cap)) {
-		int mds_wanted = __ceph_caps_mds_wanted(ci);
+		int mds_wanted = __ceph_caps_mds_wanted(ci, true);
 		int issued = __ceph_caps_issued(ci, NULL);
 
 		dout("open %p fmode %d want %s issued %s using existing\n",