Btrfs-progs: fsck: reduce memory usage for extents check

Btrfs-progs: fsck: reduce memory usage for extents check

Steps to reproduce:

 # mkfs.btrfs -f /dev/sda9 -b 2g
 # mount /dev/sda9 /mnt
 # dd if=/dev/zero of=/mnt/data bs=4k oflag=direct
 # btrfs file df /mnt

    Data, single: total=1.66GiB, used=1.66GiB
    System, single: total=4.00MiB, used=16.00KiB
    Metadata, single: total=200.00MiB, used=67.88MiB

For a filesystem without snapshots, 70M metadata, extent
checking eats max memory about 110M, this is a nightmare
for some system with low memory.

It is very likely that extent record can be freed quickly
for a filesystem without snapshots, improve this by trying
if it can free memory after adding data/tree backrefs.

This patch reduces max memory cost from 110M to 40M for
extents checking for the above case.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>