Allow arbitrary read-only shared pfn-remapping too

The VM layer (for historical reasons) turns a read-only shared mmap into
a private-like mapping with the VM_MAYWRITE bit clear.  Thus checking
just VM_SHARED isn't actually sufficient.

So use a trivial helper function for the cases where we wanted to inquire
if a mapping was COW-like or not.

Moo!

Signed-off-by: Linus Torvalds <torvalds@osdl.org>