ftrace: Add context level recursion bit checking

Currently for recursion checking in the function tracer, ftrace
tests a task_struct bit to determine if the function tracer had
recursed or not. If it has, then it will will return without going
further.

But this leads to races. If an interrupt came in after the bit
was set, the functions being traced would see that bit set and
think that the function tracer recursed on itself, and would return.

Instead add a bit for each context (normal, softirq, irq and nmi).

A check of which context the task is in is made before testing the
associated bit. Now if an interrupt preempts the function tracer
after the previous context has been set, the interrupt functions
can still be traced.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>