btrfs: return void from btrfs_add_inode_defrag()

The potential memory allocation failure is not a fatal error, skipping
autodefrag is fine and the caller inode_should_defrag() does not care
about the errors.  Further writes can attempt to add the inode back to
the defragmentation list again.

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