Truncate FAT filesystem label until 1st occurance of a blank (0x20)

Truncate FAT filesystem label until 1st occurance of a blank (0x20)

According to spec the FAT filesystem label is not terminated by a
\0-byte but instead has a fixed length where the name is padded by
blanks.
Before libblkid-tiny just passed the always 11 bytes long blank padded
string through, however it makes matching for a filesystem label
inconsistent with other filesystems, which use a \0-terminated string.

Before: "foobar     "
After : "foobar"

Signed-off-by: Mirko Vogt <mirko-openwrt@nanl.de>