lisafs: Use uint64 for the NumBytes bounds check in PReadResp.CheckedUnmarshal.

[arcusbuilds]

This fixes a panic in lisafs when a malformed PReadResp frame contains a NumBytes value with high bits set.

The root cause was a bounds check that compared NumBytes after truncating it to uint32, while the later slice used the full uint64 value. That allowed invalid lengths to pass validation and triggered a slice-bounds panic.

Fixes #14424

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[elassasmohamed8-prog]

Confirmed this addresses the root cause reported in #14424 exactly: the uint32-truncated comparison admitted values like 0x0000000C00000000 (truncates to 0) while the subsequent slice used the full uint64.

For regression coverage, this minimal test reproduces the panic on the pre-fix tree and passes with this change (both verified on a local build):

func TestReproPReadRespNumBytesTruncation(t *testing.T) {
	in := []byte("\x00\x00\x00\x00\x0c\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00")
	var m PReadResp
	defer func() {
		if r := recover(); r != nil {
			t.Fatalf("panic: %v", r)
		}
	}()
	m.CheckedUnmarshal(in)
}

Happy to push this as a commit onto this branch if maintainers would like it included.