FAT replace compare_sign with strncmp.

The static function compare_sign is only used to compare the fs_type string
and does not do anything more than what strncmp does.

The addition of the trailing '\0' to fs_type, while legal, is not needed
because the it is never printed out and strncmp does not depend on NULL
terminated strings.

Signed-off-by: Tom Rix <Tom.Rix@windriver.com>