Btrfs: use a slab for the free space entries

Since we alloc/free free space entries a whole lot, lets use a slab to keep
track of them.  This makes some of my tests slightly faster.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

diff --cc fs/btrfs/ctree.h
Simple merge

diff --cc fs/btrfs/free-space-cache.c
Simple merge

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