[SPARC64]: 8-byte align return value from compat_alloc_user_space()

Otherwise we get a ton of unaligned exceptions, for cases such
as compat_sys_msgrcv() which go:

	p = compat_alloc_user_space(second + sizeof(struct msgbuf));

and here 'second' can for example be an arbitrary odd value.

Based upon a bug report from Jurij Smakov.

Signed-off-by: David S. Miller <davem@davemloft.net>