mm: remove prev check from do_vmi_align_munmap()

If the prev does not exist, the vma iterator will be set to MAS_NONE,
which will be treated as a MAS_START when the mas_next or mas_find is
used.  In this case, the next caller will be the vma iterator, which
uses mas_find() under the hood and will now do what the user expects.

Link: https://lkml.kernel.org/r/20230724183157.3939892-5-Liam.Howlett@oracle.com
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/mmap.c

@@ -2459,8 +2459,6 @@ do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
 	}
 
 	prev = vma_prev(vmi);
-	if (unlikely((!prev)))
-		vma_iter_set(vmi, start);
 
 	/*
 	 * Detach a range of VMAs from the mm. Using next as a temp variable as