nfsd: Eliminate an allocation in nfs4_make_rec_clidname()

Since MD5 digests are fixed-size, make nfs4_make_rec_clidname() store
the digest in a stack buffer instead of a dynamically allocated buffer.
Use MD5_DIGEST_SIZE instead of a hard-coded value, both in
nfs4_make_rec_clidname() and in the definition of HEXDIR_LEN.

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