prctl: exe link permission error changed from -EINVAL to -EPERM

This brings consistency with the rest of the prctl() syscall where
-EPERM is returned when failing a capability check.

Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
Signed-off-by: Adrian Reber <areber@redhat.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Link: https://lore.kernel.org/r/20200719100418.2112740-7-areber@redhat.com


Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>