nfsd4: fix use of op_share_access

NFSv4.1 adds additional flags to the share_access argument of the open
call.  These flags need to be masked out in some of the existing code,
but current code does that inconsistently.

Tested-by: Michael Groshans <groshans@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>