ext4: avoid unnecessary spreading of allocations among groups

mb_set_largest_free_order() updates lists containing groups with largest
chunk of free space of given order. The way it updates it leads to
always moving the group to the tail of the list. Thus allocations
looking for free space of given order effectively end up cycling through
all groups (and due to initialization in last to first order). This
spreads allocations among block groups which reduces performance for
rotating disks or low-end flash media. Change
mb_set_largest_free_order() to only update lists if the order of the
largest free chunk in the group changed.

Fixes: 196e402adf2e ("ext4: improve cr 0 / cr 1 group scanning")
CC: [email protected]
Reported-and-tested-by: Stefan Wahren <[email protected]>
Tested-by: Ojaswin Mujoo <[email protected]>
Reviewed-by: Ritesh Harjani (IBM) <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

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