Currently mb_optimize_scan scan feature which improves filesystem
performance heavily (when FS is fragmented), seems to be not working
with files with extents (ext4 by default has files with extents).

This patch fixes that and makes mb_optimize_scan feature work
for files with extents.

Below are some performance numbers obtained when allocating a 10M and 100M
file with and w/o this patch on a filesytem with no 1M contiguous block.

<perf numbers>
===============
Workload: dd if=/dev/urandom of=test conv=fsync bs=1M count=10/100

Time taken
=====================================================
no.     Size   without-patch     with-patch    Diff(%)
1       10M      0m8.401s         0m5.623s     33.06%
2       100M     1m40.465s        1m14.737s    25.6%

<debug stats>
=============
w/o patch:
  mballoc:
    reqs: 17056
    success: 11407
    groups_scanned: 13643
    cr0_stats:
            hits: 37
            groups_considered: 9472
            useless_loops: 36
            bad_suggestions: 0
    cr1_stats:
            hits: 11418
            groups_considered: 908560
            useless_loops: 1894
            bad_suggestions: 0
    cr2_stats:
            hits: 1873
            groups_considered: 6913
            useless_loops: 21
    cr3_stats:
            hits: 21
            groups_considered: 5040
            useless_loops: 21
    extents_scanned: 417364
            goal_hits: 3707
            2^n_hits: 37
            breaks: 1873
            lost: 0
    buddies_generated: 239/240
    buddies_time_used: 651080
    preallocated: 705
    discarded: 478

with patch:
  mballoc:
    reqs: 12768
    success: 11305
    groups_scanned: 12768
    cr0_stats:
            hits: 1
            groups_considered: 18
            useless_loops: 0
            bad_suggestions: 0
    cr1_stats:
            hits: 5829
            groups_considered: 50626
            useless_loops: 0
            bad_suggestions: 0
    cr2_stats:
            hits: 6938
            groups_considered: 580363
            useless_loops: 0
    cr3_stats:
            hits: 0
            groups_considered: 0
            useless_loops: 0
    extents_scanned: 309059
            goal_hits: 0
            2^n_hits: 1
            breaks: 1463
            lost: 0
    buddies_generated: 239/240
    buddies_time_used: 791392
    preallocated: 673
    discarded: 446

Fixes: 196e402a (ext4: improve cr 0 / cr 1 group scanning)
Cc: stable@kernel.org
Reported-by: Geetika Moolchandani <Geetika.Moolchandani1@ibm.com>
Reported-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Suggested-by: Ritesh Harjani <riteshh@linux.ibm.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/fc9a48f7f8dcfc83891a8b21f6dd8cdf056ed810.1646732698.git.ojaswin@linux.ibm.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>