NFSv4: Convert nfs_alloc_seqid() to return an ERR_PTR() if allocation fails

When we relax the sequencing on the NFSv4.1 OPEN/CLOSE code, we will want
to use the value NULL to indicate that no sequencing is needed.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>