sh: fix futex FUTEX_OP_SET op on userspace addresses

Commit 00b73d8d ("sh: add working futex atomic ops on userspace
addresses for smp") changed the futex_atomic_op_inuser function to
use a loop. In case of the FUTEX_OP_SET op with a userspace address
containing a value different of 0, this loop is an endless loop.

Fix that by loading the value of oldval from the userspace before doing
the cmpxchg op, also for the FUTEX_OP_SET case.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Rich Felker <dalias@libc.org>