LU-14361 statahead: add statahead advise IOCTL

LU-14361 statahead: add statahead advise IOCTL

This patch reuse ioctl(LL_IOC_LADVISE2) for statahead advise.
This allows userspace programs to advise the kernel statahead
of the order that they will be traversing a directory, so that
the client can prefetch inode attributes from the MDT, similar
to what posix_fadvise(POSIX_FADV_SEQUENTIAL) does for file data.

After patched mdtest via adding this statahead IOCTL hint, it
can support mdtest benchmark with regularized file naming format:
	mdtest.$rank.$i
The usage of this statahead advise IOCTL could be as follows:
	open(dir);
	ioctl(dir_fd, IOC_LADVISE2, ...);
	stat mdtest.0.0;
	stat mdtest.0.1;
	stat mdtest.0.2;
	stat mdtest.0.3;
	...
	clsoedir(dir);

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Iac38e33bfc6d7a0b755c2646ba8053a263e3afc9
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48625
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>