sh: Handle -ERESTART_RESTARTBLOCK for restartable syscalls.

The current implementation only handles -ERESTARTNOHAND, whereas we
also need to handle -ERESTART_RESTARTBLOCK in the handle_signal()
case for restartable system calls.

As noted by Carl:

This fixes the LTP test nanosleep03 - the current kernel causes
-ERESTART_RESTARTBLOCK to reach user space rather than the correct
-EINTR.

Reported-by: Carl Shaw <shaw.carl@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>