Replaced deprecated f_vfsmnt

f_vfsmnt was removed in 3.9 kernel (and was #defined as f_path.mnt in older kernels)
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=182be684784334598eee1d90274e7f7aa0063616
Tested on 3.11.0-12 2.6.38-8