btrfs: remove btrfs_bio::fs_info by extracting it from btrfs_bio::inode

btrfs: remove btrfs_bio::fs_info by extracting it from btrfs_bio::inode

Currently there is only one caller which doesn't populate
btrfs_bio::inode, and that's scrub.

The idea is scrub doesn't want any automatic csum verification nor
read-repair, as everything will be handled by scrub itself.

However that behavior is really no different than metadata inode, thus
we can reuse btree_inode as btrfs_bio::inode for scrub.

The only exception is in btrfs_submit_chunk() where if a bbio is from
scrub or data reloc inode, we set rst_search_commit_root to true.
This means we still need a way to distinguish scrub from metadata, but
that can be done by a new flag inside btrfs_bio.

Now btrfs_bio::inode is a mandatory parameter, we can extract fs_info
from that inode thus can remove btrfs_bio::fs_info to save 8 bytes from
btrfs_bio structure.

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