btrfs: replace nested usage of min & max with clamp in btrfs_compress_set_level()

Refactor the btrfs_compress_set_level() function by replacing the
nested usage of min() and max() macro with clamp() to simplify the
code and improve readability.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: George Hu <integral@archlinux.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>