btrfs: Allow to add new compression algorithm

Make the code aware of compression type, instead of always assuming
zlib compression.

Also make the zlib workspace function as common code for all
compression types.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

fs/btrfs/extent_map.c

@@ -3,6 +3,7 @@
 #include <linux/module.h>
 #include <linux/spinlock.h>
 #include <linux/hardirq.h>
+#include "ctree.h"
 #include "extent_map.h"
 
 
@@ -54,6 +55,7 @@ struct extent_map *alloc_extent_map(gfp_t mask)
 		return em;
 	em->in_tree = 0;
 	em->flags = 0;
+	em->compress_type = BTRFS_COMPRESS_NONE;
 	atomic_set(&em->refs, 1);
 	return em;
 }