rb_copy_generic_ivar: alloc the proper shape capacity #7326

Job	Run time
cargo test	45s
cargo clippy	28s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev, /usr/lib/ll...	4m 33s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	8m 2s
make (check, --enable-yjit=dev)	4m 49s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	5m 11s
make (test-bundled-gems, --enable-yjit=dev)	4m 48s
make (yjit-bench, --enable-yjit=dev, --yjit-stats, true)	5m 59s
YJIT Ubuntu result	1s
	34m 36s

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