nfsd41: expand solo sequence check

Compounds consisting of only a sequence operation don't need any
additional caching beyond the sequence information we store in the slot
entry.  Fix nfsd4_is_solo_sequence to identify this case correctly.

The additional check for a failed sequence in nfsd4_store_cache_entry()
is redundant, since the nfsd4_is_solo_sequence call lower down catches
this case.

The final ce_cachethis set in nfsd4_sequence is also redundant.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>