Fix off-by-one error in validation of write_mostly.
The user-supplied value given for the 'write_mostly' argument must be an
index starting at 0. The validation of the supplied argument failed to
check for 'N' ('>' vs '>='), which would have caused an access beyond the
end of the array.
Reported-by: Doug Ledford <dledford@redhat.com> Signed-off-by: Jonathan Brassow <jbrassow@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>