ftrace_graph_caller was never run after calling ftrace_trace_function,
breaking the function graph tracer. Fix this, bringing it in line with the
x86 implementation.

While we're at it, also streamline the control flow of _mcount a bit to
reduce the number of branches.

This issue was reported before:
https://www.linux-mips.org/archives/linux-mips/2014-11/msg00295.htmlSigned-off-by: NMatthias Schiffer <mschiffer@universe-factory.net>
Tested-by: NMatt Redfearn <matt.redfearn@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/18929/Signed-off-by: NPaul Burton <paul.burton@mips.com>
Cc: stable@vger.kernel.org # v3.17+