pkeys: Remove easily triggered WARN

This easy-to-trigger warning shows up instantly when running
Trinity on a kernel with CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS disabled.

At most this should have been a printk, but the -EINVAL alone should be more
than adequate indicator that something isn't available.

Signed-off-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>