NFSD: Hoist rq_vec preparation into nfsd_read()

Accrue the following benefits:

a) Deduplicate this common bit of code.

b) Don't prepare rq_vec for NFSv2 and NFSv3 spliced reads, which
   don't use rq_vec. This is already the case for
   nfsd4_encode_read().

c) Eventually, converting NFSD's read path to use a bvec iterator
   will be simpler.

In the next patch, nfsd_iter_read() will replace nfsd_readv() for
all NFS versions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>