lib/strn*,objtool: Enforce user_access_begin() rules

Apparently GCC can fail to inline a 'static inline' single caller
function:

  lib/strnlen_user.o: warning: objtool: strnlen_user()+0x33: call to do_strnlen_user() with UACCESS enabled
  lib/strncpy_from_user.o: warning: objtool: strncpy_from_user()+0x33: call to do_strncpy_from_user() with UACCESS enabled

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220408094718.262932488@infradead.org