Btrfs: use args in the correct order for kcalloc in btrfsic_read_block

kcalloc is defined as:

  kcalloc(size_t n, size_t size, gfp_t flags)

Although this won't cause problems in practice, btrfsic_read_block()
uses kcalloc with n and size in the opposite order.

Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>