fat/nls: Fix handling of utf8 invalid char

With utf8 option, vfat allowed the duplicated filenames.

Normal nls returns -EINVAL for invalid char. But utf8s_to_utf16s()
skipped the invalid char historically.

So, this changes the utf8s_to_utf16s() directly to return -EINVAL for
invalid char, because vfat is only user of it.

mkdir /mnt/fatfs
FILENAME=`echo -ne "invalidutf8char_\\0341_endofchar"`
echo "Using filename: $FILENAME"
dd if=/dev/zero of=fatfs bs=512 count=128
mkdosfs -F 32 fatfs
mount -o loop,utf8 fatfs /mnt/fatfs
touch "/mnt/fatfs/$FILENAME"
umount /mnt/fatfs
mount -o loop,utf8 fatfs /mnt/fatfs
touch "/mnt/fatfs/$FILENAME"
ls -l /mnt/fatfs
umount /mnt/fatfs

----  And the output is:

Using filename: invalidutf8char_\0341_endofchar
128+0 records in
128+0 records out
65536 bytes (66 kB) copied, 0.000388118 s, 169 MB/s
mkdosfs 2.11 (12 Mar 2005)
total 0
-rwxr-xr-x 1 root root 0 Jun 28 19:46 invalidutf8char__endofchar
-rwxr-xr-x 1 root root 0 Jun 28 19:46 invalidutf8char__endofchar

Tested-by: Marton Balint <cus@fazekas.hu>
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>