ARM: 7468/1: ftrace: Trace function entry before updating index

Commit 722b3c74695377d11d18a52f3da08114d37f3f37 modified x86 ftrace to
avoid tracing all functions called from irqs when function graph was
used with a filter.  Port the same fix to ARM.

Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>