btrfs: mark extent buffer alignment checks as unlikely

We are not expecting to ever fail the extent buffer alignment checks, so
mark them as unlikely to allow the compiler to potentially generate more
optimized code.

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