ceph: EIO all operations after forced umount

This patch makes try_get_cap_refs() and __do_request() check
if the file system was forced umount, and return -EIO if it was.
This patch also adds a helper function to drops dirty caps and
wakes up blocking operation.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
diff --git a/fs/ceph/super.c b/fs/ceph/super.c
index d1c833c..69e6e19 100644
--- a/fs/ceph/super.c
+++ b/fs/ceph/super.c
@@ -708,6 +708,7 @@
 	if (!fsc)
 		return;
 	fsc->mount_state = CEPH_MOUNT_SHUTDOWN;
+	ceph_mdsc_force_umount(fsc->mdsc);
 	return;
 }