LU-17383 statahead: quit statahead with a long time wait

LU-17383 statahead: quit statahead with a long time wait

If the thread is not doing stat for more than a time threshold
(@sbi->ll_sa_timeout, 30 seconds by default) then it probably does
not care too much about performance, or is no longer using this
directory.
Quit the statahead thread with a long time wait in this case.

This patch also fixes defects reported by Coverity Scan for
Lustre.

Fixes: e10bf68d7c3 ("LU-14361 statahead: regularized fname statahead pattern")
Test-Parameters: testlist=parallel-scale-nfsv4
Test-Parameters: testlist=parallel-scale-nfsv4
Test-Parameters: testlist=parallel-scale-nfsv4
Test-Parameters: testlist=parallel-scale-nfsv4
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Ia7c478268fe12eeefa6dfae1b3c94451f010d1d5
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53535
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>