btrfs: remove btrfs_raid_bio::fs_info member

We can grab fs_info reliably from btrfs_raid_bio::bioc, as the bioc is
always passed into alloc_rbio(), and only get released when the raid bio
is released.

Remove btrfs_raid_bio::fs_info member, and cleanup all the @fs_info
parameters for alloc_rbio() callers.

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