Allocate the lcb in slave_alloc and free it in slave_destroy.  This allows
us to remove all the code that checks to see if it's already been allocated.

From: Christoph Hellwig <hch@lst.de>
Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>