btrfs: use a kmem_cache for free space control structures

We are currently allocating the free space control structures for block
groups using the generic slabs, and given that the size of the
btrfs_free_space_ctl structure is 152 bytes (on a release kernel), we end
up using the kmalloc-192 slab and therefore waste quite some memory since
on a 4K page system we can only fit 21 free space control structures per
page. These structures are allocated and delallocated every time we create
and remove block groups.

So use a kmem_cache for free space control structures, this way on a 4K
page system we can fit 26 structures instead of 21.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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