msdosfs(5): Reflect READONLY attribute in file mode

Msdosfs allows setting READONLY by clearing the owner write bit of the file
mode. (While here, correct the misspelling of S_IWUSR as VWRITE. No
functional change.)

In msdosfs_getattr, intuitively reflect that READONLY attribute to userspace
in the file mode.

Reported by: Karl Denninger <karl AT denninger.net>
Sponsored by: Dell EMC Isilon