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>

include/linux/pkeys.h

@@ -25,7 +25,6 @@ static inline int mm_pkey_alloc(struct mm_struct *mm)
 
 static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
 {
-	WARN_ONCE(1, "free of protection key when disabled");
 	return -EINVAL;
 }