urandom: save seed even if random pool size is less than 4096 bits

Linux 5.18 replaced the LFSR used for entropy extraction with BLAKE2s,
which reduced the entropy pool size from 4096 to 256 bits.
init.d/urandom extracts the seed to save in 512-byte chunks, rounding down
when calculating the number of chunks to save, resulting in no random seed
being saved on these kernels. Fix the init script by reading a single
variable-sized chunk from /dev/urandom.

Signed-off-by: Hunor Csordás <csordas.hunor@gmail.com>

--- openrc-0.44.10/init.d/urandom.in.orig	2022-06-10 12:58:17.432472085 +0200
+++ openrc-0.44.10/init.d/urandom.in	2022-06-10 13:15:59.263851886 +0200
@@ -21,15 +21,15 @@
 
 save_seed()
 {
-	local psz=1
+	local psz=512
 
 	if [ -e /proc/sys/kernel/random/poolsize ]; then
-		: $(( psz = $(cat /proc/sys/kernel/random/poolsize) / 4096 ))
+		: $(( psz = $(cat /proc/sys/kernel/random/poolsize) / 8 ))
 	fi
 
 	(	# sub shell to prevent umask pollution
 		umask 077
-		dd if=/dev/urandom of="$urandom_seed" count=${psz} 2>/dev/null
+		dd if=/dev/urandom of="$urandom_seed" bs=${psz} count=1 2>/dev/null
 	)
 }