x86/uaccess: Don't leak AC flag into fentry from mcsafe_handle_tail()

After adding mcsafe_handle_tail() to the objtool uaccess safe list,
objtool reports:

arch/x86/lib/usercopy_64.o: warning: objtool: mcsafe_handle_tail()+0x0: call to __fentry__() with UACCESS enabled

With SMAP, this function is called with AC=1, so it needs to be careful
about which functions it calls. Disable the ftrace entry hook, which
can potentially pull in a lot of extra code.

Signed-off-by: Josh Poimboeuf <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lkml.kernel.org/r/[e-mail]

arch/x86/lib/usercopy_64.c

···
  * but reuse __memcpy_mcsafe in case a new read error is encountered.
  * clac() is handled in _copy_to_iter_mcsafe().
  */
-__visible unsigned long
+__visible notrace unsigned long
 mcsafe_handle_tail(char *to, char *from, unsigned len)
 {
 	for (; len; --len, to++, from++) {