ext4: fixed potential NULL dereference in ext4_calculate_overhead()

The memset operation before check can cause a BUG if the memory
allocation failed.  Since we are using get_zeroed_age, there is no
need to use memset anyway.

Found by the Spruce system in cooperation with the KEDR Framework.
Signed-off-by: NVahram Martirosyan <vmartirosyan@linuxtesting.org>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -3206,7 +3206,6 @@ int ext4_calculate_overhead(struct super_block *sb)
 	ext4_fsblk_t overhead = 0;
 	char *buf = (char *) get_zeroed_page(GFP_KERNEL);
 
-	memset(buf, 0, PAGE_SIZE);
 	if (!buf)
 		return -ENOMEM;