btrfs: add an fs_info parameter for compression workspace manager

[BACKGROUND]
Currently btrfs shares workspaces and their managers for all filesystems,
this is mostly fine as all those workspaces are using page size based
buffers, and btrfs only support block size (bs) <= page size (ps).

This means even if bs < ps, we at most waste some buffer space in the
workspace, but everything will still work fine.

The problem here is that is limiting our support for bs > ps cases.

As now a workspace now may need larger buffer to handle bs > ps cases,
but since the pool has no way to distinguish different workspaces, a
regular workspace (which is still using buffer size based on ps) can be
passed to a btrfs whose bs > ps.

In that case the buffer is not large enough, and will cause various
problems.

[ENHANCEMENT]
To prepare for the per-fs workspace migration, add an fs_info parameter
to all workspace related functions.

For now this new fs_info parameter is not yet utilized.

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