btrfs: constify tracepoint arguments

Tracepoint arguments are all read-only.  If we mark the arguments
as const, we're able to keep or convert those arguments to const
where appropriate.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index d87ac27..31c259c 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -207,7 +207,7 @@ struct btrfs_inode {
 
 extern unsigned char btrfs_filetype_table[];
 
-static inline struct btrfs_inode *BTRFS_I(struct inode *inode)
+static inline struct btrfs_inode *BTRFS_I(const struct inode *inode)
 {
 	return container_of(inode, struct btrfs_inode, vfs_inode);
 }
@@ -231,7 +231,7 @@ static inline void btrfs_insert_inode_hash(struct inode *inode)
 	__insert_inode_hash(inode, h);
 }
 
-static inline u64 btrfs_ino(struct btrfs_inode *inode)
+static inline u64 btrfs_ino(const struct btrfs_inode *inode)
 {
 	u64 ino = inode->location.objectid;