Add _Static_asserts to check that file system fid structures are not larger than struct fid

[rmacklem]

Summary

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.

Test Plan

Compiled the kernel with these _Static_assert()s.