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

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 <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/8e13d6f0da1c8a3f7603903da6cbf6d582bbfe10.1563413318.git.jpoimboe@redhat.com