sparc64: Do not clobber %g2 in xcall_fetch_glob_regs().

%g2 is meant to hold the CPUID number throughout this routine, since
at the very beginning, and at the very end, we use %g2 to calculate
indexes into per-cpu arrays.

However we erroneously clobber it in order to hold the %cwp register
value mid-stream.

Fix this code to use %g3 for the %cwp read and related calulcations
instead.

Reported-by: Meelis Roos <mroos@linux.ee>
Signed-off-by: David S. Miller <davem@davemloft.net>