If we should happen to get an altivec assist exception while executing in
the kernel, we will currently try to handle it and fail, and end up oopsing
with (apparently) a segfault.  (An altivec assist exception occurs for
floating-point altivec instructions with denormalized inputs or outputs if
the altivec unit is in java mode.)

This patch checks explicitly if we are in user mode and prints a useful
message if not.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>