CVE-2022-49280: Integer Underflow (Wrap or Wraparound)

In the Linux kernel, the following vulnerability has been resolved:

NFSD: prevent underflow in nfssvc_decode_writeargs()

Smatch complains:

fs/nfsd/nfsxdr.c:341 nfssvc_decode_writeargs() warn: no lower bound on 'args->len'

Change the type to unsigned to prevent this issue.