ext4: use prandom_u32() instead of get_random_bytes()

Many of the uses of get_random_bytes() do not actually need
cryptographically secure random numbers.  Replace those uses with a
call to prandom_u32(), which is faster and which doesn't consume
entropy from the /dev/random driver.
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ialloc.c

@@ -432,7 +432,7 @@ static int find_group_orlov(struct super_block *sb, struct inode *parent,
 			ext4fs_dirhash(qstr->name, qstr->len, &hinfo);
 			grp = hinfo.hash;
 		} else
-			get_random_bytes(&grp, sizeof(grp));
+			grp = prandom_u32();
 		parent_group = (unsigned)grp % ngroups;
 		for (i = 0; i < ngroups; i++) {
 			g = (parent_group + i) % ngroups;

fs/ext4/mmp.c

@@ -259,7 +259,7 @@ static unsigned int mmp_new_seq(void)
 	u32 new_seq;
 
 	do {
-		get_random_bytes(&new_seq, sizeof(u32));
+		new_seq = prandom_u32();
 	} while (new_seq > EXT4_MMP_SEQ_MAX);
 
 	return new_seq;

fs/ext4/super.c

@@ -3068,7 +3068,6 @@ static struct ext4_li_request *ext4_li_request_new(struct super_block *sb,
 {
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	struct ext4_li_request *elr;
-	unsigned long rnd;
 
 	elr = kzalloc(sizeof(*elr), GFP_KERNEL);
 	if (!elr)
@@ -3083,10 +3082,8 @@ static struct ext4_li_request *ext4_li_request_new(struct super_block *sb,
 	 * spread the inode table initialization requests
 	 * better.
 	 */
-	get_random_bytes(&rnd, sizeof(rnd));
-	elr->lr_next_sched = jiffies + (unsigned long)rnd %
-			     (EXT4_DEF_LI_MAX_START_DELAY * HZ);
-
+	elr->lr_next_sched = jiffies + (prandom_u32() %
+				(EXT4_DEF_LI_MAX_START_DELAY * HZ));
 	return elr;
 }