mappings as if MAP_SHARED was always present since in general MAP_PRIVATE
is not permitted for character devices. However, there is one exception
in that MAP_PRIVATE mappings are permitted for /dev/zero.
Only require a writable file descriptor (FWRITE) for shared, writable
mappings of character devices. vm_mmap_cdev() will reject any private
mappings for other devices.
Reviewed by: kib
Reported by: sbruno (broke qemu cross-builds), peter
Differential Revision: https://reviews.freebsd.org/D3316
fada4adf95