ceph: fail the open_by_handle_at() if the dentry is being unlinked

When unlinking a file the kclient will send a unlink request to MDS
by holding the dentry reference, and then the MDS will return 2 replies,
which are unsafe reply and a deferred safe reply.

After the unsafe reply received the kernel will return and succeed
the unlink request to user space apps.

Only when the safe reply received the dentry's reference will be
released. Or the dentry will only be unhashed from dcache. But when
the open_by_handle_at() begins to open the unlinked files it will
succeed.

The inode->i_count couldn't be used to check whether the inode is
opened or not.

Link: https://tracker.ceph.com/issues/56524
Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Luís Henriques <lhenriques@suse.de>
Tested-by: Luís Henriques <lhenriques@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>