random: use lockless method of accessing and updating f->reg_idx

commit 92e75428 upstream.

Linus pointed out that there is a much more efficient way of avoiding
the problem that we were trying to address in commit 9dfa7bba:
"fix race in drivers/char/random.c:get_reg()".

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>