LU-8372 llite: Fix suspicious dereference of pointer 'vma->vm_file'

LU-8372 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).

Test-Parameters: trivial

Change-Id: I309127fae03a77020883a0e190c4fa6ea341dfee
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: http://review.whamcloud.com/21171
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>