btrfs: fix uninitialized return value from btrfs_reclaim_sweep()

The return variable 'ret' at btrfs_reclaim_sweep() is never assigned if
none of the space infos is reclaimable (for example if periodic reclaim
is disabled, which is the default), so we return an undefined value.

This can be fixed my making btrfs_reclaim_sweep() not return any value
as well as do_reclaim_sweep() because:

1) do_reclaim_sweep() always returns 0, so we can make it return void;

2) The only caller of btrfs_reclaim_sweep() (btrfs_reclaim_bgs()) doesn't
   care about its return value, and in its context there's nothing to do
   about any errors anyway.

Therefore remove the return value from btrfs_reclaim_sweep() and
do_reclaim_sweep().

Fixes: e4ca3932 ("btrfs: periodic block_group reclaim")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>