mount.nfs: Use correct data type in discover_nfs_mount_data_version()

Address compiler warning:

    mount.c: In function discover_nfs_mount_data_version¿:
    mount.c:162: warning: comparison between signed and unsigned
    mount.c:164: warning: comparison between signed and unsigned
    mount.c:166: warning: comparison between signed and unsigned
    mount.c:168: warning: comparison between signed and unsigned
    mount.c:170: warning: comparison between signed and unsigned
    mount.c:178: warning: comparison between signed and unsigned

linux_version_code() and MAKE_VERSION() both return an unsigned int.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>