btrfs: make close_ctree return void

There's no user of the return value and we can get rid of the comment in
put_super.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

diff --cc fs/btrfs/disk-io.c
Simple merge

diff --cc fs/btrfs/disk-io.h
Simple merge

diff --cc fs/btrfs/super.c
Simple merge