sparc32: Fix unaligned stack handling on trap return.

When the rett stack checking code sees the stack is unaligned (in both
the sun4c and srmmu cases) it jumps to the window fault-in path.

But that just tries to page the stack pages in, it doesn't do anything
special if the stack is misaligned.

Therefore we essentially just loop forever in the trap return path.

Fix this by emitting a SIGILL in the stack fault-in code if the stack
is mis-aligned.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>