btrfs: constify structs with op functions or static definitions

There are some op tables that can be easily made const, similarly the
sysfs feature and raid tables. This is motivated by PaX CONSTIFY plugin.

Signed-off-by: David Sterba <dsterba@suse.cz>

diff --cc fs/btrfs/compression.c
Simple merge

diff --cc fs/btrfs/compression.h
Simple merge

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/extent_io.h
Simple merge

diff --cc fs/btrfs/lzo.c
Simple merge

diff --cc fs/btrfs/props.c
Simple merge

diff --cc fs/btrfs/sysfs.c
Simple merge

diff --cc fs/btrfs/transaction.c
Simple merge

diff --cc fs/btrfs/volumes.c
Simple merge

diff --cc fs/btrfs/zlib.c
Simple merge