kernel/panic.c: display reason at end + pr_emerg

Currently, booting without initrd specified on 80x25 screen gives a call
trace followed by atkbd : Spurious ACK.  Original message ("VFS: Unable
to mount root fs") is not available.  Of course this could happen in
other situations...

This patch displays panic reason after call trace which could help lot
of people even if it's not the very last line on screen.

Also, convert all panic.c printk(KERN_EMERG to pr_emerg(

[akpm@linux-foundation.org: missed a couple of pr_ conversions]
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>