Fix alloc/free of cache item

xfs_extfree_item_cache is allocated and freed twice. Remove the
obsolete alloc/free.

Signed-off-by: Torsten Rupp <torsten.rupp@gmx.net>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>