vboxsf: Fix the check for the old binary mount-arguments struct

Fix the check for the mainline vboxsf code being used with the old
mount.vboxsf mount binary from the out-of-tree vboxsf version doing
a comparison between signed and unsigned data types.

This fixes the following smatch warnings:

fs/vboxsf/super.c:390 vboxsf_parse_monolithic() warn: impossible condition '(options[1] == (255)) => ((-128)-127 == 255)'
fs/vboxsf/super.c:391 vboxsf_parse_monolithic() warn: impossible condition '(options[2] == (254)) => ((-128)-127 == 254)'
fs/vboxsf/super.c:392 vboxsf_parse_monolithic() warn: impossible condition '(options[3] == (253)) => ((-128)-127 == 253)'

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>