ACPI: thinkpad-acpi: allow for syscall restart in sysfs handlers

Map an mutex_lock_interruptible() error return into ERESTARTSYS, as the
only possible error from mutex_lock_interruptible is EINTR, and that will
only happen if signal_pending() causes the mutex lock attempt to abort.

This still allows signals to be delivered ASAP, which is much nicer than
just doing mutex_lock, and still shadows userspace from EINTR when
SA_RESTART is active.

Problem reported by Peter Jordan.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Jean Delvare <khali@linux-fr.org>
Cc: Peter Jordan <usernetwork@gmx.info>
Cc: Richard Neill <rn214@hermes.cam.ac.uk>
Signed-off-by: Len Brown <len.brown@intel.com>