btrfs: add btrfs_reserve_data_bytes and use it

Create a new function btrfs_reserve_data_bytes() in order to handle data
reservations.  This uses the new flush types and flush states to handle
making data reservations.

This patch specifically does not change any functionality, and is
purposefully not cleaned up in order to make bisection easier for the
future patches.  The new helper is identical to the old helper in how it
handles data reservations.  We first try to force a chunk allocation,
and then we run through the flush states all at once and in the same
order that they were done with the old helper.

Subsequent patches will clean this up and change the behavior of the
flushing, and it is important to keep those changes separate so we can
easily bisect down to the patch that caused the regression, rather than
the patch that made us start using the new infrastructure.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>