btrfs: drop transaction parameter from btrfs_add_inode_defrag()

There's only one caller inode_should_defrag() that passes NULL to
btrfs_add_inode_defrag() so we can drop it an simplify the code.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>