btrfs: use slab for end_io_wq structures

The structure is frequently reused.  Rename it according to the slab
name.

Signed-off-by: David Sterba <dsterba@suse.cz>

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