rust: zerocopy: update to v0.8.54

rust: zerocopy: update to v0.8.54

Update our vendored copy of `zerocopy` (and `zerocopy-derive`) to v0.8.54.

It is a very small delta from v0.8.52, and most importantly it resolves
the unexpected lack of inlining [1] which triggered a modpost error
under `CONFIG_CC_OPTIMIZE_FOR_SIZE=y` reported by Alexandre using Gary's
suggestion [2]:

    ERROR: modpost: "_RNvMNtCs5wX7wwEUCR9_8zerocopy6layoutNtB2_8SizeInfo24try_to_nonzero_elem_size" [drivers/gpu/nova-core.ko] undefined!
    ERROR: modpost: "_RNvNtCs5wX7wwEUCR9_8zerocopy4util18padding_needed_for" [drivers/gpu/nova-core.ko] undefined!

It also resolves `most_traits` being unexpectedly documented [3] that I
reported and adds a missing SPDX license identifier [4] that I requested
to match the kernel version.

The following script may be used to check for the remaining differences:

    for path in $(cd rust/zerocopy-derive/ && find . -type f ! -name README.md); do
        curl --silent --show-error --location \
            https://github.com/google/zerocopy/raw/v0.8.54/zerocopy/zerocopy-derive/src/$path |
            git diff --no-index - rust/zerocopy-derive/$path &&
            echo $path: OK
    done

    for path in $(cd rust/zerocopy/ && find . -type f ! -name README.md); do
        curl --silent --show-error --location \
            https://github.com/google/zerocopy/raw/v0.8.54/zerocopy/$path |
            git diff --no-index - rust/zerocopy/$path &&
            echo $path: OK
    done

Cc: Joshua Liebow-Feeser <joshlf@google.com>
Cc: Jack Wrenn <jswrenn@google.com>
Reported-by: Alexandre Courbot <acourbot@nvidia.com>
Closes: https://lore.kernel.org/rust-for-linux/20260708-zerocopy-export-v1-1-2bfc355853c6@nvidia.com/ [1]
Suggested-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/rust-for-linux/DJT6235B3DOV.222XR5O6VHG4M@garyguo.net/
Link: https://github.com/google/zerocopy/issues/3466
Link: https://github.com/google/zerocopy/issues/3457
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260709211311.142544-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>