passthrough_ll: add some of the missing operations

passthrough_ll: add some of the missing operations

New operations added:

 - mkdir
 - mknod
 - symlink
 - link
 - unlink
 - rmdir
 - rename
 - setattr
 - fsyncdir
 - flush
 - fsync
 - statfs
 - fallocate

Caveats:

 - The utimes(2) family of syscalls will fail on symlinks on 4.18 and
   earlier kernels.  Hoping to add support to later kernels.

 - The link(2) and linkat(2) system calls will fail on symlinks unless running
   with privileges (CAP_DAC_READ_SEARCH).

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>