net: always set reason in sk_filter_trim_cap()

net: always set reason in sk_filter_trim_cap()

sk_filter_trim_cap() will soon return the drop reason by value.

Make sure *reason is cleared when no error is returned,
to ease this conversion.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-7 (-7)
Function                                     old     new   delta
sk_filter_trim_cap                           889     882      -7
Total: Before=29722668, After=29722661, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260409145625.2306224-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>