ftrace: use preempt_enable/disable notrace macros to avoid double fault

Since the backport commit eea46baf ("ftrace: Fix possible
use-after-free issue in ftrace_location()") on linux-5.4.y branch, the
old ftrace_int3_handler()->ftrace_location() path has included
rcu_read_lock(), which has mcount location inside and leads to potential
double fault.

Replace rcu_read_lock/unlock with preempt_enable/disable notrace macros
so that the mcount location does not appear on the int3 handler path.

This fix is specific to linux-5.4.y branch, the only branch still using
ftrace_int3_handler with commit e60b613df8b6 ("ftrace: Fix possible
use-after-free issue in ftrace_location()") backported. It also avoids
the need to backport the code conversion to text_poke() on this branch.

Reported-by: Koichiro Den <koichiro.den@canonical.com>
Closes: https://lore.kernel.org/all/74gjhwxupvozwop7ndhrh7t5qeckomt7yqvkkbm5j2tlx6dkfk@rgv7sijvry2k


Fixes: eea46baf ("ftrace: Fix possible use-after-free issue in ftrace_location()") # linux-5.4.y
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>