LU-12988 ldiskfs: port ext4-mballoc-prefetch.patch to RHEL 8.1

LU-12988 ldiskfs: port ext4-mballoc-prefetch.patch to RHEL 8.1

ext4: mballoc to prefetch groups ahead of scanning

Prefetching is done in 8 * flex_bg groups, so it should be 8 read-ahead
reads for a single allocating thread. At the end of allocation the
thread waits for read-ahead completion and initializes buddy information
so that read-aheads are not lost in case of memory pressure.

At cr=0 the number of prefetching IOs is limited per allocation context
to prevent a situation when mballoc loads thousands of bitmaps looking
for a perfect group and ignoring groups with good chunks.

Change-Id: I34e3e3dd604379fb65d711b620e8b30ae5b7df6e
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Lustre-bug-id: https://jira.whamcloud.com/browse/LU_12988
Reviewed-on: https://review.whamcloud.com/37811
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>