sparc64: Fix constraints on swab helpers.

[ Upstream commit 5a2b59d3993e8ca4f7788a48a23e5cb303f26954 ]

We are reading the memory location, so we have to have a memory
constraint in there purely for the sake of showing the data flow
to the compiler.

Reported-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>