mbuf: Don't force all M_EXTPG mbufs to be read-only

Some M_EXTPG mbufs are read-only (e.g. those backing sendfile
requests), but others are not. Add a flags argument to
mb_alloc_ext_pgs that can be used to set M_RDONLY when needed rather
than setting it unconditionally. Update mb_unmapped_to_ext to
preserve M_RDONLY from the unmapped mbuf.

Reviewed by: gallatin
Differential Revision: https://reviews.freebsd.org/D46783