LU-16338 readahead: add stats for read-ahead page count

LU-16338 readahead: add stats for read-ahead page count

This patch adds the stats for read-ahead page count:

lctl get_param llite.*.read_ahead_stats
llite.lustre-ffff938b7849d000.read_ahead_stats=
snapshot_time             4011.320890492 secs.nsecs
start_time                0.000000000 secs.nsecs
elapsed_time              4011.320890492 secs.nsecs
hits                      4 samples [pages]
misses                    1 samples [pages]
zero_size_window          4 samples [pages]
failed_to_reach_end       1 samples [pages]
failed_to_fast_read       1 samples [pages]
readahead_pages           1 samples [pages] 255 255 255

Lustre-change: https://review.whamcloud.com/49224
Lustre-commit: cdcf97e17e73dfdd65c4e46bb30c4a07f5e710cf

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Iada06eb7d78ab28cfcc7167e49d25da252da4009
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <farr0186@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/50070
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>