btrfs: separate superblock items out of fs_info

fs_info has now ~9kb, more than fits into one page. This will cause
mount failure when memory is too fragmented. Top space consumers are
super block structures super_copy and super_for_commit, ~2.8kb each.
Allocate them dynamically. fs_info will be ~3.5kb. (measured on x86_64)

Add a wrapper for freeing fs_info and all of it's dynamically allocated
members.

Signed-off-by: David Sterba <dsterba@suse.cz>
diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 8ec5d86..14f1c5a 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -85,7 +85,8 @@
 static inline int compressed_bio_size(struct btrfs_root *root,
 				      unsigned long disk_size)
 {
-	u16 csum_size = btrfs_super_csum_size(&root->fs_info->super_copy);
+	u16 csum_size = btrfs_super_csum_size(root->fs_info->super_copy);
+
 	return sizeof(struct compressed_bio) +
 		((disk_size + root->sectorsize - 1) / root->sectorsize) *
 		csum_size;