ftrace, x86: rename in_nmi variable

Impact: clean up

The in_nmi variable in x86 arch ftrace.c is a misnomer.
Andrew Morton pointed out that the in_nmi variable is incremented
by all CPUS. It can be set when another CPU is running an NMI.

Since this is actually intentional, the fix is to rename it to
what it really is: "nmi_running"

Signed-off-by: Steven Rostedt <srostedt@redhat.com>