btrfs-progs: misc-tests/038: fix wrong field filtered under root directory

Ran misc-tests/038 in /root/btrfs-progs:

  make test-misc TEST=038\*
      [TEST]   misc-tests.sh
      [TEST/misc]   038-backup-root-corruption
  ./test.sh: line 33: [: bytenr=65536,: integer expression expected
  Backup slot 2 is not in use
  test failed for case 038-backup-root-corruption
  make: *** [Makefile:401: test-misc] Error 1

It's caused by the wrong line filtered by
$(dump_super | grep root | head -n1 | awk '{print $2}').

The command $(dump-super | grep root) outputs:

  superblock: bytenr=65536, device=/root/btrfs-progs/tests/test.img
  root                    30605312
  chunk_root_generation   5
  root_level              0
  chunk_root              22036480
  chunk_root_level        0
  log_root                0
  log_root_transid        0
  log_root_level          0
  root_dir                6
  backup_roots[4]:

Here
"superblock: bytenr=65536, device=/root/btrfs-progs/tests/test.img" is
selected but not the line "root                    30605312".

Restricting the awk rule can fix it.

Fixes: 78a3831d46 ("btrfs-progs: tests: Test backup root retention logic")
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Su Yue <Damenly_Su@gmx.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/misc-tests/038-backup-root-corruption/test.sh

@@ -23,7 +23,7 @@ dump_super() {
 # Ensure currently active backup slot is the expected one (slot 3)
 backup2_root_ptr=$(dump_super | grep -A1 "backup 2" | grep backup_tree_root | awk '{print $2}')
 
-main_root_ptr=$(dump_super | grep root | head -n1 | awk '{print $2}')
+main_root_ptr=$(dump_super | awk '/^root\t/{print $2}')
 
 [ "$backup2_root_ptr" -eq "$main_root_ptr" ] || _fail "Backup slot 2 is not in use"