mm: thp: use down_read_trylock() in khugepaged to avoid long block

In the current design, khugepaged needs to acquire mmap_sem before
scanning an mm.  But in some corner cases, khugepaged may scan a process
which is modifying its memory mapping, so khugepaged blocks in
uninterruptible state.  But the process might hold the mmap_sem for a
long time when modifying a huge memory space and it may trigger the
below khugepaged hung issue:

  INFO: task khugepaged:270 blocked for more than 120 seconds.
  Tainted: G E 4.9.65-006.ali3000.alios7.x86_64 #1
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  khugepaged D 0 270 2 0x00000000 
  ffff883f3deae4c0 0000000000000000 ffff883f610596c0 ffff883f7d359440
  ffff883f63818000 ffffc90019adfc78 ffffffff817079a5 d67e5aa8c1860a64
  0000000000000246 ffff883f7d359440 ffffc90019adfc88 ffff883f610596c0
  Call Trace:
    schedule+0x36/0x80
    rwsem_down_read_failed+0xf0/0x150
    call_rwsem_down_read_failed+0x18/0x30
    down_read+0x20/0x40
    k...