Initialize 'bc_rdonly' to the right value.

Note that independent of this change a readonly disk file would still be
opened O_RDONLY and protected from writes by the guest.

Reviewed by:	grehan


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271439 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f