mm/truncate: add parameter explanation for invalidate_mapping_pagevec

mm/truncate: add parameter explanation for invalidate_mapping_pagevec

To fix a kernel-doc markups issue:

  mm/truncate.c:646: warning: Function parameter or member 'mapping' not described in 'invalidate_mapping_pagevec'
  mm/truncate.c:646: warning: Function parameter or member 'start' not described in 'invalidate_mapping_pagevec'
  mm/truncate.c:646: warning: Function parameter or member 'end' not described in 'invalidate_mapping_pagevec'
  mm/truncate.c:646: warning: Function parameter or member 'nr_pagevec' not described in 'invalidate_mapping_pagevec'

Link: https://lkml.kernel.org/r/1605605088-30668-1-git-send-email-alex.shi@linux.alibaba.com
Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>