ARM: prevent multiple syscall restarts

Al Viro reports that calling "sys_sigsuspend(-ERESTARTNOHAND, 0, 0)"
with two signals coming and being handled in kernel space results
in the syscall restart being done twice.

Avoid this by clearing the 'why' flag when we call the signal handling
code to prevent further syscall restarts after the first.

Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>