device-dax: fail all private mapping attempts

device-dax: fail all private mapping attempts

The device-dax implementation originally tried to be tricky and allow
private read-only mappings, but in the process allowed writable
MAP_PRIVATE + MAP_NORESERVE mappings.  For simplicity and predictability
just fail all private mapping attempts since device-dax memory is
statically allocated and will never support overcommit.

Cc: <[email protected]>
Cc: Dave Hansen <[email protected]>
Fixes: dee410792419 ("/dev/dax, core: file operations and dax-mmap")
Reported-by: Pawel Lebioda <[email protected]>
Signed-off-by: Dan Williams <[email protected]>