Btrfs: remove unnecessary key copy when logging inode

The btrfs_insert_empty_item() function doesn't modify its
key argument.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Zach Brown <zab@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>