s390/hypfs_diag: Diag204 busy loop

When diag204 busy-indiciation facility is installed and diag204 is
returning busy, hypfs diag204 handler now does an interruptable busy
wait until diag204 is no longer busy. If there is a signal pending, call
would be restarted with -ERESTARTSYSCALL, except for fatal signals.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>