btrfs: Remove fs_info argument from btrfs_trans_release_metadata

All current callers of this function just get a reference to the
trans->fs_info member and pass it as the second argument. Collapse this
into the function itself. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>