dm suspend: return -ERESTARTSYS instead of -EINTR

This commit changes device mapper, so that it returns -ERESTARTSYS
instead of -EINTR when it is interrupted by a signal (so that the ioctl
can be restarted).

The manpage signal(7) says that the ioctl function should be restarted if
the signal was handled with SA_RESTART.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org