MFC 196399: fstat: fix fsid comparison when executed on systems with 64-bit long

MFC 196399: fstat: fix fsid comparison when executed on systems with 64-bit long

This affects only fstat on zfs and devfs, only on 64-bit systems
and only when fsid is greater than 2^31 - 1.
When fstat examines a file via stat(2) it takes uint32_t st_dev
and assigns to (signed) (64-bit) long fsid, this results in
a positive value.
When fstat examines opened files it takes int32_t f_fsid.val[0]
and assigns to (signed) (64-bit) long fsid, this results in
a negative value.
So, while initially st_dev and f_fsid.val[0] have the same bit
values they get promoted to different 64-bit values because
of the signed-vs-unsigned difference.

A fix is to use "more natural" positive numbers by introducing
intermediate unsigned cast for f_fsid.val[0].

Reviewed by:    jhb, lulf
Approved by:    re (kib)
MFC after:      1 week (to stable/7)

git-svn-id: svn://svn.freebsd.org/base/stable/8@196400 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f