btrfs: add fs context handling functions

We are going to use the fs context to hold the mount options, so
allocate the btrfs_fs_context when we're asked to init the fs context,
and free it in the free callback.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>