exit: Fix Sparse errors and warnings

This patch fixes the following sparse error:
kernel/exit.c:627:25: error: incompatible types in comparison expression

And the following warning:
kernel/exit.c:626:40: warning: incorrect type in assignment

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: edit commit message]
Link: https://lore.kernel.org/r/20200130062028.4870-1-madhuparnabhowmik10@gmail.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>