wifi: brcmfmac: fweh: Fix boot crash on Raspberry Pi 4

Fix boot crash on Raspberry Pi by moving the update to `event->datalen`
before data is copied into flexible-array member `data` via `memcpy()`.

Flexible-array member `data` was annotated with `__counted_by(datalen)`
in commit 62d19b358088 ("wifi: brcmfmac: fweh: Add __counted_by for
struct brcmf_fweh_queue_item and use struct_size()"). The intention of
this is to gain visibility into the size of `data` at run-time through
its _counter_ (in this case `datalen`), and with this have its accesses
bounds-checked at run-time via CONFIG_FORTIFY_SOURCE and
CONFIG_UBSAN_BOUNDS.

To effectively accomplish the above, we shall update the counter
(`datalen`), before the first access to the flexible array (`data`),
which was also done in the mentioned commit.

However, commit edec42821911 ("wifi: brcmfmac: allow per-vendor event
handling") inadvertently caused a buffer overflow, detected by
FORTIFY_SOURCE. It moved the `event->datalen = datalen;` update to after
the first `data` access, at which point `event->datalen` was not yet
updated from zero (after calling `kzalloc()`), leading to the overflow
issue.

This fix repositions the `event->datalen = datalen;` update before
accessing `data`, restoring the intended buffer overflow protection. :)

Fixes: edec42821911 ("wifi: brcmfmac: allow per-vendor event handling")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://gist.github.com/nathanchance/e22f681f3bfc467f15cdf6605021aaa6
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/Zc+3PFCUvLoVlpg8@neat

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