hf-api: knock 2 kiB off stack usage

hf-api includes a function that computes a CRC32 of a chunk of data.
Unfortunately, it made the (easy, common) mistake of allocating a
crc::Crc on the stack.

The upstream crc crate, despite our protests, optimizes for time over
space. A crc::Crc includes a lookup table to accelerate the calculation
of the CRC. Creating one at runtime on the stack means

1. The lookup table is on your stack (all 2048 bytes of it, in this
   case), and

2. Your function now contains code to generate the lookup table.

This is a footgun in the crc API design, IMO. It's far too easy to make
this mistake.

Anyway, made it static for a 2 kiB improvement in stack margin.

drv/hf-api/src/lib.rs

@@ -166,8 +166,8 @@ impl HfRawPersistentData {
     }
 
     fn expected_checksum(&self) -> u32 {
-        let c = crc::Crc::<u32>::new(&crc::CRC_32_ISCSI);
-        let mut c = c.digest();
+        static CRC: crc::Crc<u32> = crc::Crc::<u32>::new(&crc::CRC_32_ISCSI);
+        let mut c = CRC.digest();
         // We do a CRC32 of everything except the checksum, which is positioned
         // at the end of the struct and is a `u32`
         let size = core::mem::size_of::<HfRawPersistentData>()