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>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-arch@vger.kernel.org
Cc: arnd@arndb.de
Cc: linux-api@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: luto@kernel.org
Cc: torvalds@linux-foundation.org
Cc: akpm@linux-foundation.org
Cc: mgorman@techsingularity.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>