btrfs: reorganize logic at free_extent_buffer() for better readability

It's hard to read the logic to break out of the while loop since it's a
very long expression consisting of a logical or of two composite
expressions, each one composed by a logical and. Further each one is also
testing for the EXTENT_BUFFER_UNMAPPED bit, making it more verbose than
necessary.

So change from this:

    if ((!test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) && refs <= 3)
        || (test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags) &&
            refs == 1))
       break;

To this:

    if (test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags)) {
        if (refs == 1)
            break;
    } else if (refs <= 3) {
            break;
    }

At least on x86_64 using gcc 9.3.0, this doesn't change the object size.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>