ftrace: convert ftrace_lock from a spinlock to mutex

Impact: clean up

The older versions of ftrace required doing the ftrace list
search under atomic context. Now all the calls are in non-atomic
context. There is no reason to keep the ftrace_lock as a spinlock.

This patch converts it to a mutex.

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