tmpfs: Rework file handles

Change 'struct tmpfs_fid_data' to behave consistently with the private
structure other FSes use. In a nutshell, make it a full alias of
'struct fid', instead of just using it to fill 'fid_data'. This implies
adding a length field at start (aliasing 'fid_len' of 'struct fid'), and
filling 'fid_len' with the full size of the aliased structure.

To ensure that the new 'struct tmpfs_fid_data' is smaller than 'struct
fid', which the compile-time assert introduced in commit
91b5592a1e1af974 ("fs: Add static asserts for the size of fid
structures") checks (and thus was not strong enough when added), use
'__packed'.

A consequence of this change is that copying the 'struct tmpfs_fid_data'
into a stack-allocated variable becomes unnecessary, we simply rely on
the compiler emitting the proper code on seeing '__packed' (and on the
start of 'struct tmpfs_fid_data' being naturally aligned, which is
normally guaranteed by kernel's malloc() and/or inclusion in 'struct
fhandle').

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47956