fs: Remove i_nlink check from file system link callback

Now that VFS check for inode->i_nlink == 0 and returns proper
error, remove similar check from file system

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>