sparc64: Fix top-level fault handling bugs.

sparc64: Fix top-level fault handling bugs.

[ Upstream commit 70ffc6ebaead783ac8dafb1e87df0039bb043596 ]

Make get_user_insn() able to cope with huge PMDs.

Next, make do_fault_siginfo() more robust when get_user_insn() can't
actually fetch the instruction.  In particular, use the MMU announced
fault address when that happens, instead of calling
compute_effective_address() and computing garbage.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>