x86/mm/pat: Ensure different messages in STRICT_DEVMEM and PAT cases

STRICT_DEVMEM and PAT produce same failure accessing /dev/mem,
which is quite confusing to the user. Make printk messages
different to lessen confusion.

Signed-off-by: Pavel Machek <pavel@ucw.cz>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>