vfs: stop always overwriting ->mnt_stat in VFS_STATFS

The struct is already populated on each mount (and remount). Fields are either
constant or not used by filesystem in the first place.

Some infrequently used functions use it to avoid having to allocate a new buffer
and are left alone.

The current code results in an avoidable copying single-threaded and significant
cache line bouncing multithreaded

While here deduplicate initial filling of the struct.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21317