btrfs: make inode_can_compress take btrfs_inode

Gets rid of superfluous BTRFS_I() calls.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>