Fix bogus FREAD with NFSV4OPEN_ACCESSREAD. No functional change.

The code in nfscl_doflayoutio() bogusly used FREAD instead of
NFSV4OPEN_ACCESSREAD. Since both happen to be defined as "1", this
worked and the patch doesn't result in a functional change.
Found by inspection during development of Flex File Layout support.

MFC after: 2 weeks