remove allocation in nfs_fh3

[Vaiz]

nfs_fh3 type currently uses Cow<'_, [u8]>, which results in a significant memory footprint, nearly equivalent to using a fixed [u8; 64]. The primary downside of using Cow is that the owned variant requires heap allocations, introducing unnecessary overhead.

According to the NFSv3 specification:

   nfs_fh3
      struct nfs_fh3 {
         opaque       data<NFS3_FHSIZE>;
      };

   NFS3_FHSIZE 64
      The maximum size in bytes of the opaque file handle.

Since NFS3_FHSIZE is a fixed 64-byte limit, it may be more efficient to replace Cow<'_, [u8]> with [u8; 64] or another more memory-efficient representation, avoiding dynamic allocations where possible.