LU-12988 ldiskfs: mballoc to prefetch groups

LU-12988 ldiskfs: mballoc to prefetch groups

ahead of scanning. prefething 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 allocating 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.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ibce9a060900544abe994f4d7a20ac9c2979ccc56
Reviewed-on: https://review.whamcloud.com/37633
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>