arm64: function_graph: Remove use of FTRACE_NOTRACE_DEPTH

Functions in the set_graph_notrace no longer subtract FTRACE_NOTRACE_DEPTH
from curr_ret_stack, as that is now implemented via the trace_recursion
flags. Access to curr_ret_stack no longer needs to worry about checking for
this. curr_ret_stack is still initialized to -1, when there's not a shadow
stack allocated.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>