btrfs: replace sb::s_blocksize by fs_info::sectorsize

[ Upstream commit 4e00422e ]

The block size stored in the super block is used by subsystems outside
of btrfs and it's a copy of fs_info::sectorsize. Unify that to always
use our sectorsize, with the exception of mount where we first need to
use fixed values (4K) until we read the super block and can set the
sectorsize.

Replace all uses, in most cases it's fewer pointer indirections.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 46a6e10a ("btrfs: send: allow cloning non-aligned extent if it ends at i_size")
Signed-off-by: Sasha Levin <sashal@kernel.org>