mm/mmap: remove unnecessary assignment

pgoff is not used after the statement "pgoff = vma->vm_pgoff;", so the
assignment is redundant.

Signed-off-by: Yanchuan Nian <ycnian@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mmap.c

@@ -1566,7 +1566,6 @@ munmap_back:
 		WARN_ON_ONCE(addr != vma->vm_start);
 
 		addr = vma->vm_start;
-		pgoff = vma->vm_pgoff;
 		vm_flags = vma->vm_flags;
 	} else if (vm_flags & VM_SHARED) {
 		error = shmem_zero_setup(vma);