staging/lustre/llite: Fix suspicious dereference of pointer 'vma->vm_file'

Remove useless LASSERT(vma->vm_file) because of if it's NULL it
will crash early in file_inode(vma->vm_file).

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/21171
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-8372


Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>