SPARC supports 32 bit and 64 bit xchg right now. Add the support
for 16 bit (2 byte) xchg. This is required to support queued spinlock
feature which uses 2 byte xchg. This is achieved using 4 byte cas
instructions with byte manipulations.

Also re-arranged the code to call __cmpxchg_u32 inside xchg16.
Signed-off-by: NBabu Moger <babu.moger@oracle.com>
Reviewed-by: NHåkon Bugge <haakon.bugge@oracle.com>
Reviewed-by: NSteven Sistare <steven.sistare@oracle.com>
Reviewed-by: NShannon Nelson <shannon.nelson@oracle.com>
Reviewed-by: NJane Chu <jane.chu@oracle.com>
Reviewed-by: NVijay Kumar <vijay.ac.kumar@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>