NFSv4: Replace the open coded decode_opaque_inline() with the new generic

Also ensure that we always check that the size of the decoded object
matches the expectation that it must be smaller than NFS4_OPAQUE_LIMIT.
This should be true for all the current users of decode_opaque_inline(),
including decode_ace(), decode_pathname(), decode_attr_fs_locations()
and decode_exchange_id().

Note that this allows us to get rid of a number of existing checks in
decode_exchange_id(),

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>