btrfs: introduce BTRFS_MAX_ITEM_SIZE

We use BTRFS_LEAF_DATA_SIZE - sizeof(struct btrfs_item) in
several places.  This introduces a BTRFS_MAX_ITEM_SIZE macro to do the
same.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 5e409e2..104a2e4 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4573,8 +4573,7 @@
 	btrfs_set_fs_incompat(info, RAID56);
 }
 
-#define BTRFS_MAX_DEVS(r) ((BTRFS_LEAF_DATA_SIZE(r)		\
-			- sizeof(struct btrfs_item)		\
+#define BTRFS_MAX_DEVS(r) ((BTRFS_MAX_ITEM_SIZE(r)		\
 			- sizeof(struct btrfs_chunk))		\
 			/ sizeof(struct btrfs_stripe) + 1)