nfsd4: fix share mode permissions

NFSv4 opens may function as locks denying other NFSv4 users the rights
to open a file.

We're requiring a user to have write permissions before they can deny
write.  We're *not* requiring a user to have write permissions to deny
read, which is if anything a more drastic denial.

What was intended was to require write permissions for DENY_READ.

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