fs: Add static asserts for the size of fid structures

File system specific *fid structures are copied into the generic
struct fid defined in sys/mount.h.
As such, they cannot be larger than struct fid.

This patch adds _Static_assert()s to check for this.

ZFS and fuse already have _Static_assert()s.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47936