random: use a tighter cap in credit_entropy_bits_safe()

[ Upstream commit 9f886f4d

 ]

This fixes a harmless UBSAN where root could potentially end up
causing an overflow while bumping the entropy_total field (which is
ignored once the entropy pool has been initialized, and this generally
is completed during the boot sequence).

This is marginal for the stable kernel series, but it's a really
trivial patch, and it fixes UBSAN warning that might cause security
folks to get overly excited for no reason.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Chen Feng <puck.chen@hisilicon.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>