msdosfs: fixes for Undefined Behavior.

These were found by the Undefined Behaviour GsoC project at NetBSD:

Do not change signedness bit with left shift.
While there avoid signed integer overflow.
Address both issues with using unsigned type.

msdosfs_fat.c:512:42, left shift of 1 by 31 places cannot be represented
in type 'int'
msdosfs_fat.c:521:44, left shift of 1 by 31 places cannot be represented
in type 'int'
msdosfs_fat.c:744:14, left shift of 1 by 31 places cannot be represented
in type 'int'
msdosfs_fat.c:744:24, signed integer overflow: -2147483648 - 1 cannot be
represented in type 'int [20]'
msdosfs_fat.c:840:13, left shift of 1 by 31 places cannot be represented
in type 'int'
msdosfs_fat.c:840:36, signed integer overflow: -2147483648 - 1 cannot be
represented in type 'int [20]'

Detected with micro-UBSan in the user mode.

Hinted from: NetBSD (CVS 1.33)
MFC after: 2 weeks
Differenctial Revision: https://reviews.freebsd.org/D16615