ext4: keep "prefetch_grp" and "nr" consistent

Keep "prefetch_grp" and "nr" consistent to avoid to call
ext4_mb_prefetch_fini with non-prefetched groups.
When we step into next criteria, "prefetch_grp" is set to prefetch start
of new criteria while "nr" is number of the prefetched group in previous
criteria. If previous criteria and next criteria are both inexpensive
(< CR_GOAL_LEN_SLOW) and prefetch_ios reachs sbi->s_mb_prefetch_limit
in previous criteria, "prefetch_grp" and "nr" will be inconsistent and
may introduce unexpected cost to do ext4_mb_init_group for non-prefetched
groups.
Reset "nr" to 0 when we reset "prefetch_grp" to goal group to keep them
consistent.

Signed-off-by: Kemeng Shi <[email protected]>
Reviewed-by: Ojaswin Mujoo <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

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