ext4: fix EFBIG edge case when writing to large non-extent file

By running the following reproducer, we can confirm that the write 
system call returns with 0 when it should return the error EFBIG.

#!/bin/sh

/bin/dd if=/dev/zero of=./img bs=1k count=1 seek=1024k > /dev/null 2>&1
/sbin/mkfs.ext3 -Fq ./img
/bin/mount -o loop -t ext4 ./img /mnt
/bin/touch /mnt/file
strace /bin/dd if=/dev/zero of=/mnt/file conv=notrunc bs=1k count=1 seek=$((2194719883264/1024)) 2>&1 | /bin/egrep "write.* 1024\) = "
/bin/umount /mnt
exit

Signed-off-by: Toshiyuki Okajima <toshi.okajima@jp.fujitsu.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Eric Sandeen <sandeen@redhat.com>

fs/ext4/file.c

@@ -70,7 +70,8 @@ ext4_file_write(struct kiocb *iocb, const struct iovec *iov,
 		struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
 		size_t length = iov_length(iov, nr_segs);
 
-		if (pos > sbi->s_bitmap_maxbytes)
+		if ((pos > sbi->s_bitmap_maxbytes ||
+		    (pos == sbi->s_bitmap_maxbytes && length > 0)))
 			return -EFBIG;
 
 		if (pos + length > sbi->s_bitmap_maxbytes) {