The extent_io_tree is embedded in several structures, notably in struct
btrfs_inode.  The fs_info is only used for reporting errors and for
reference in trace points. We can get to the pointer through the inode,
but not all io trees set it. However, we always know the owner and
can recognize if inode is valid.  For access helpers are provided, const
variant for the trace points.

This reduces size of extent_io_tree by 8 bytes and following structures
in turn:

- btrfs_inode		1104 -> 1088
- btrfs_device		 520 ->  512
- btrfs_root		1360 -> 1344
- btrfs_transaction	 456 ->  440
- btrfs_fs_info		3600 -> 3592
- reloc_control		1520 -> 1512
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>