Use ftrace_hash instead of a static array of a fixed size.  This is
useful when a graph filter pattern matches to a large number of
functions.  Now hash lookup is done with preemption disabled to protect
from the hash being changed/freed.

Link: http://lkml.kernel.org/r/20170120024447.26097-3-namhyung@kernel.orgSigned-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>