btrfs: reduce size of struct btrfs_ref

We can reduce two members' size that in turn reduce size of struct
btrfs_ref from 64 to 56 bytes. As the structure is often used as a local
variable several functions reduce their stack usage.

- make enum btrfs_ref_type packed, there are only 4 values

- switch action and its values to a packed enum

Final structure layout:

struct btrfs_ref {
        enum btrfs_ref_type        type;                 /*     0     1 */
        enum btrfs_delayed_ref_action action;            /*     1     1 */
        bool                       skip_qgroup;          /*     2     1 */

        /* XXX 5 bytes hole, try to pack */

        u64                        bytenr;               /*     8     8 */
        u64                        len;                  /*    16     8 */
        u64                        parent;               /*    24     8 */
        union {
                struct btrfs_data_ref data_ref;          /*    32    24 */
                struct btrfs_tree_ref tree_ref;          /*    32    16 */
        };                                               /*    32    24 */

        /* size: 56, cachelines: 1, members: 7 */
        /* sum members: 51, holes: 1, sum holes: 5 */
        /* last cacheline: 56 bytes */
};

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/delayed-ref.h

@@ -9,10 +9,16 @@
 #include <linux/refcount.h>
 
 /* these are the possible values of struct btrfs_delayed_ref_node->action */
-#define BTRFS_ADD_DELAYED_REF    1 /* add one backref to the tree */
-#define BTRFS_DROP_DELAYED_REF   2 /* delete one backref from the tree */
-#define BTRFS_ADD_DELAYED_EXTENT 3 /* record a full extent allocation */
-#define BTRFS_UPDATE_DELAYED_HEAD 4 /* not changing ref count on head ref */
+enum btrfs_delayed_ref_action {
+	/* Add one backref to the tree */
+	BTRFS_ADD_DELAYED_REF = 1,
+	/* Delete one backref from the tree */
+	BTRFS_DROP_DELAYED_REF,
+	/* Record a full extent allocation */
+	BTRFS_ADD_DELAYED_EXTENT,
+	/* Not changing ref count on head ref */
+	BTRFS_UPDATE_DELAYED_HEAD,
+} __packed;
 
 struct btrfs_delayed_ref_node {
 	struct rb_node ref_node;
@@ -183,7 +189,7 @@ enum btrfs_ref_type {
 	BTRFS_REF_DATA,
 	BTRFS_REF_METADATA,
 	BTRFS_REF_LAST,
-};
+} __packed;
 
 struct btrfs_data_ref {
 	/* For EXTENT_DATA_REF */
@@ -223,7 +229,7 @@ struct btrfs_tree_ref {
 
 struct btrfs_ref {
 	enum btrfs_ref_type type;
-	int action;
+	enum btrfs_delayed_ref_action action;
 
 	/*
 	 * Whether this extent should go through qgroup record.