msdosfs: use vfs_timestamp() to generate timestamps instead of getnanotime().

Most filesystems, with the notable exceptions of msdosfs and autofs use
only vfs_timestamp() to read the current time. This has the benefit of
configurable granularity (using the vfs.timestamp_precision sysctl).

For convenience, use it on msdosfs too.

Submitted by: Damjan Jovanovic
Differential Revision: https://reviews.freebsd.org/D15297