nfsd: Replace open-coded conversion of bytes to hex

Since the Linux kernel's sprintf() has conversion to hex built-in via
"%*phN", delete md5_to_hex() and just use that.  Also add an explicit
array bound to the dname parameter of nfs4_make_rec_clidname() to make
its size clear.  No functional change.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>