btrfs: move block perfect compression out of experimental features

Commit 1d2fbb7f ("btrfs: allow compression even if the range is not
page aligned") introduced the block perfect compression for block size <
page size cases.

Before that commit, if the fs block size is smaller than page size (aka
subpage cases), compressed write is only enabled if the dirty range is
fully page aligned.

This block perfect compression support was introduced in v6.13, and has
been tested for two kernel releases.
I believe it's time to move it out of experimental features so that we
can get more tests in the real world.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>