mm/rmap: correct some obsolete comments of anon_vma

mm/rmap: correct some obsolete comments of anon_vma

commit bbc076ca20df ("mm: convert anon_vma->lock to a mutex") changed
spinlock used to serialize access to vma list to mutex.  And further, the
commit ce1bf3625288 ("mm/rmap: Convert the struct anon_vma::mutex to an
rwsem") converted the mutex to an rwsem for solving scalability problem.
So replace spinlock with rwsem to make comment uptodate.

Link: https://lkml.kernel.org/r/20210123072459.25903-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>