btrfs: remove redundant log root assignment from log_dir_items()

At log_dir_items() we are assigning the exact same value to the local
variable 'log', once when it's declared and once again shortly after.
Remove the later assignment as it's pointless.

This patch is part of a patchset comprised of the following 5 patches:

  btrfs: remove root argument from btrfs_log_inode() and its callees
  btrfs: remove redundant log root assignment from log_dir_items()
  btrfs: factor out the copying loop of dir items from log_dir_items()
  btrfs: insert items in batches when logging a directory when possible
  btrfs: keep track of the last logged keys when logging a directory

This is patch 2/5. The change log of the last patch (5/5) has performance
results.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>